make/esptool: fix FFLAGS inclusion order for qemu

[aabadie]

Contribution description

This PR fixes the inclusion order of FFLAGS with the esptool when using the esp_qemu module. In master, this is currently broken (probably because of #15475).

The fix proposed by this PR is to move the logic from esp32 to esptool.inc.mk, which is now included after the cpu Makefile.include is included.

This PR is a quick fix and IMHO a more proper fix is to reuse the EMULATE=1 variable and integrate the esp emulator into the qemu.inc.mk mechanism, instead of relying on the module mechanism. I leave this as a follow-up.

Another cleanup that is worth is to get rid of all this commands configured in the FFLAGS variable, this is an abuse of this variable that should only contain parameters of the flasher tool (configured in FLASHER). I'm also leaving this as a follow-up.

Testing procedure

• Try to flash an application while using USEMODULE=esp_qemu: in master this fails, with this PR it works:

this PR:

$ USEMODULE=esp_qemu make BOARD=esp32-wroom-32 -C examples/hello-world flash-only --no-print-directory 
ESP32_SDK_DIR should be defined as /path/to/esp-idf directory
ESP32_SDK_DIR is set by default to /opt/esp/esp-idf
/work/riot/RIOT/dist/tools/esptool/esptool.py --chip esp32 elf2image --flash_mode dout --flash_size 4MB --flash_freq 40m     -o /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/hello-world.elf.bin /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/hello-world.elf; printf "\n" > /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv; printf "nvs, data, nvs, 0x9000, 0x6000\n" >> /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv; printf "phy_init, data, phy, 0xf000, 0x1000\n" >> /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv; printf "factory, app, factory, 0x10000, " >> /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv; ls -l /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/hello-world.elf.bin | awk '{ print $5 }' >> /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv; python3 /work/riot/RIOT/dist/tools/esptool/gen_esp32part.py --verify /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.bin
esptool.py v2.4.0
Parsing CSV input...
dd if=/dev/zero bs=1M count=4 | tr "\\000" "\\377" > tmp.bin && cat tmp.bin | head -c $((0x1000)) | cat - /work/riot/RIOT/cpu/esp32/bin/bootloader.bin tmp.bin | head -c $((0x8000)) | cat - /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.bin tmp.bin | head -c $((0x10000)) | cat - /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/hello-world.elf.bin tmp.bin | head -c 4MB > /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/esp32flash.bin && rm tmp.bin; cp /work/riot/RIOT/cpu/esp32/bin/rom_0x3ff90000_0x00010000.bin /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/rom1.bin && cp /work/riot/RIOT/cpu/esp32/bin/rom_0x40000000_0x000c2000.bin /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/rom.bin
4+0 records in
4+0 records out
4194304 bytes (4,2 MB, 4,0 MiB) copied, 0,0091344 s, 459 MB/s

master:

$ USEMODULE=esp_qemu make BOARD=esp32-wroom-32 -C examples/hello-world flash-only --no-print-directory 
ESP32_SDK_DIR should be defined as /path/to/esp-idf directory
ESP32_SDK_DIR is set by default to /opt/esp/esp-idf
/work/riot/RIOT/dist/tools/esptool/esptool.py --chip esp32 elf2image --flash_mode dout --flash_size 4MB --flash_freq 40m     -o /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/hello-world.elf.bin /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/hello-world.elf; printf "\n" > /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv; printf "nvs, data, nvs, 0x9000, 0x6000\n" >> /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv; printf "phy_init, data, phy, 0xf000, 0x1000\n" >> /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv; printf "factory, app, factory, 0x10000, " >> /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv; ls -l /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/hello-world.elf.bin | awk '{ print $5 }' >> /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv; python3 /work/riot/RIOT/dist/tools/esptool/gen_esp32part.py --verify /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.csv /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.bin
esptool.py v2.4.0
Parsing CSV input...
dd cp /work/riot/RIOT/cpu/esp32/bin/rom_0x3ff90000_0x00010000.bin /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/rom1.bin && cp /work/riot/RIOT/cpu/esp32/bin/rom_0x40000000_0x000c2000.bin /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/rom.bin if=/dev/zero bs=1M count=4 | tr "\\000" "\\377" > tmp.bin && cat tmp.bin | head -c $((0x1000)) | cat - /work/riot/RIOT/cpu/esp32/bin/bootloader.bin tmp.bin | head -c $((0x8000)) | cat - /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/partitions.bin tmp.bin | head -c $((0x10000)) | cat - /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/hello-world.elf.bin tmp.bin | head -c 4MB > /work/riot/RIOT/examples/hello-world/bin/esp32-wroom-32/esp32flash.bin && rm tmp.bin;
dd: unrecognized operand ‘cp’
Try 'dd --help' for more information.
make: *** [/work/riot/RIOT/examples/hello-world/../../Makefile.include:714: flash-only] Error 1

=> one can see that the cp commands related to qemu are called before the other commands configured in the FFLAGS, so, right after dd, which is the flasher in this case. This is why it fails badly.

Issues/PRs references

Noticed that problem while working on #15970