west sign regression when HEX file not exists

[nandojve]

Describe the bug
west sign, as command says, should sign zephyr output files for MCUboot, for instance. When using minimal parameters west will try sign both hex and bin files. The west sign is failing on current zephyr mainline for projects that create only a bin file. This is the case of frdm-k64f board.

To Reproduce
Steps to reproduce the behavior:

1. west build -b frdm_k64f -d build/app zephyr/samples/subsys/mgmt/updatehub
2. west sign -t imgtool -d build/app -- --version 1.0.0 --pad --key bootloader/mcuboot/root-rsa-2048.pem
3. See error

FATAL ERROR: no unsigned .hex found at /home/gfbudke/zephyros/mainline/build/app/zephyr/zephyr.hex

Expected behavior
west sign should skip to sign when target file not exists. if no targets, a graceful exit is expected.

Impact
annoyance

Environment (please complete the following information):

• OS: Linux
• Toolchain Zephyr SDK 0.12.3
• Commit SHA 3eb05e5

Additional context
west sign --no-hex can be used as workaround.

west sign --no-hex -t imgtool -d build/app -- --version 1.0.0 --pad --key bootloader/mcuboot/root-rsa-2048.pem
=== image configuration:
partition offset: 131072 (0x20000)
partition size: 393216 (0x60000)
rom start offset: 512 (0x200)
=== signing binaries
unsigned bin: /home/gfbudke/zephyros/mainline/build/app/zephyr/zephyr.bin
signed bin:   /home/gfbudke/zephyros/mainline/build/app/zephyr/zephyr.signed.bin

This was discovered at #34442

[mbolivar-nordic]

This was fixed in 250b213, with a follow-up fix for a spurious warning in 2de6bf9.