cpu/esp8266: update deprecated flash_size argument

Using flash size in megabits is deprecated by esptool.
Use the new megabyte notation.

    WARNING: Flash size arguments in megabits like '8m' are deprecated.
    Please use the equivalent size '1MB'.
    Megabit arguments may be removed in a future release.
    esptool.py v2.6

cpu/esp8266/Makefile.include

@@ -132,7 +132,7 @@ LINKFLAGS += -Wl,--warn-unresolved-symbols
 FLASHFILE ?= $(ELFFILE)
 
 # configure preflasher to convert .elf to .bin before flashing
-FLASH_SIZE = -fs 8m
+FLASH_SIZE = -fs 1MB
 PREFLASHER ?= esptool.py
 PREFFLAGS  ?= elf2image $(FLASH_SIZE) $(FLASHFILE)
 FLASHDEPS  += preflash