cpu/esp32: set CONFIG_ESPTOOL_FLASH_FREQ from FLASH_FREQ

CONFIG_ESPTOOL_FLASH_FREQ_* defines are used in ESP-IDF to select the right SPI clock speed for flash.

cpu/esp32/Makefile.include

@@ -115,6 +115,19 @@ CFLAGS += -DCONFIG_IDF_TARGET_$(call uppercase_and_underscore,$(CPU_FAM))
 CFLAGS += -DESP_PLATFORM
 CFLAGS += -DLOG_TAG_IN_BRACKETS
 
+# extend CFLAGS by the corresponding FLASH_FREQ
+ifeq (20m,$(FLASH_FREQ))
+  CFLAGS += -DCONFIG_ESPTOOLPY_FLASHFREQ_20M
+else ifeq (26m,$(FLASH_FREQ))
+  CFLAGS += -DCONFIG_ESPTOOLPY_FLASHFREQ_26M
+else ifeq (40m,$(FLASH_FREQ))
+  CFLAGS += -DCONFIG_ESPTOOLPY_FLASHFREQ_40M
+else ifeq (80m,$(FLASH_FREQ))
+  CFLAGS += -DCONFIG_ESPTOOLPY_FLASHFREQ_80M
+else ifeq (120m,$(FLASH_FREQ))
+  CFLAGS += -DCONFIG_ESPTOOLPY_FLASHFREQ_120M
+endif
+
 # shortcuts used by ESP-IDF
 CFLAGS += -Dasm=__asm
 CFLAGS += -Dtypeof=__typeof__