Fix typo: IMAGEDEF → IMAGE_DEF

[vazhnov]

Fix typo: IMAGEDEF → IMAGE_DEF, as it is defined in the datasheet.

[lurch]

LGTM...

$ git grep IMAGEDEF | wc -l
2
$ git grep IMAGE_DEF | wc -l
32

🙂

@will-v-pi Looks like a similar fix will also be needed to enc_bootloader/memmap_mbedtls.ld in picotool.

[vazhnov]

@lurch, @will-v-pi, I created the change: raspberrypi/picotool#265

[kilograham]

thx