at91: sam9x,sama5: fix racy SD card image generation

at91: sam9x,sama5: fix racy SD card image generation

We've few low spec (make -j3) build workers attached to the 22.03
buildbot instance which from time to time exhibit following build
failure during image generation (shortened for brewity):

 + dd bs=512 if=root.ext4 of=openwrt-22.03...sdcard.img.gz.img
 dd: failed to open 'root.ext4': No such file or directory

Thats happening likely due to the fact, that on buildbots we've
`TARGET_PER_DEVICE_ROOTFS=y` which produces differently named filesystem
image in the SD card image target dependency chain:

 make_ext4fs -L rootfs ... root.ext4+pkg=68b329da

and that hardcoded `root.ext4` image filename becomes available from
other Make targets in the later stages. So lets fix this issue by using
IMAGE_ROOTFS Make variable which should contain proper path to the root
filesystem image.

Fixing remaining subtargets ommited in commit 5c3679e39b61 ("at91:
sama7: fix racy SD card image generation").

Fixes: 5c3679e39b61 ("at91: sama7: fix racy SD card image generation")
Signed-off-by: Petr Štetiar <ynezz@true.cz>