cpu/{esp_common,esp8266,esp32}: fix default FLASH_* configuration

This commit includes the following changes:
- the default flash mode in bootloader should be dio and not dout
- the default flash configuration for ESP32 has to be exported to be also visible in cpu/esp32/bootloader/Makefile
- the comments in  cpu/esp8266/Makefile.include have to be removed so that the flash configuration do not contain trailing white spaces

cpu/esp32/Makefile.include

@@ -1,14 +1,17 @@
 # ESP32x specific flashing options
 FLASH_CHIP  = $(CPU_FAM)
+
+# Serial flasher config as used by the ESP-IDF, be careful when overriding them.
+# They have to be exported to use same values in subsequent makefiles.
 ifeq (esp32,$(CPU_FAM))
-  FLASH_MODE ?= dout
-  FLASH_FREQ  = 40m   # DO NOT CHANGE
-  FLASH_SIZE ?= 4
+  export FLASH_MODE ?= dout
+  export FLASH_FREQ ?= 40m
+  export FLASH_SIZE ?= 2
   BOOTLOADER_POS = 0x1000
 else ifeq (esp32c3,$(CPU_FAM))
-  FLASH_MODE ?= dio
-  FLASH_FREQ  = 80m   # DO NOT CHANGE
-  FLASH_SIZE ?= 4
+  export FLASH_MODE ?= dio
+  export FLASH_FREQ ?= 80m
+  export FLASH_SIZE ?= 2
   BOOTLOADER_POS = 0x0000
 else
   $(error Unkwnown ESP32x SoC variant (family))

cpu/esp32/bootloader/Makefile

@@ -315,9 +315,9 @@ FLASH_CHIP = $(CPU_FAM)
 ESPTOOL ?= $(RIOTTOOLS)/esptools/esptool_v3.2.py
 # TODO: These should be exported/configurable from the app side. That would
 # require to export these values.
-FLASH_MODE ?= dout
-FLASH_FREQ ?= 40m
-FLASH_SIZE ?= 4
+FLASH_MODE ?= dio   # ESP-IDF uses dio as default flash mode for the bootloader
+FLASH_FREQ ?= 40m   # lowest frequency all ESP32x SoC support
+FLASH_SIZE ?= 2     # smallesrt size all ESP32x SoC support
 
 # We use esptool to extract a version 1 app from the bootloader.elf. This is
 # like the regular objdump binary file but it contains a 16 byte header which

cpu/esp8266/Makefile.include

@@ -4,10 +4,11 @@
 CFLAGS_OPT ?= -O2
 
 # ESP8266 specific flashing options
+# DO NOT CHANGE, ESP8266/ESP8285 modules only work with dout and 26m
 FLASH_CHIP  = esp8266
 FLASH_OPTS  = --version 3
-FLASH_MODE  = dout  # DO NOT CHANGE, ESP8266/ESP8285 modules only work with dout
-FLASH_FREQ  = 26m   # DO NOT CHANGE
+FLASH_MODE  = dout
+FLASH_FREQ  = 26m
 FLASH_SIZE ?= 1
 BOOTLOADER_POS = 0x0000