at91: sama7: fix racy SD card image generation

at91: sama7: 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:

 + dd bs=512 if=root.ext4 of=openwrt-22.03-snapshot-r20028-43d71ad93e-at91-sama7-microchip_sama7g5-ek-ext4-sdcard.img.gz.img seek=135168 conv=notrunc
 dd: failed to open 'root.ext4': No such file or directory

Thats 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 becomes available from other target 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.

Signed-off-by: Petr Štetiar <ynezz@true.cz>