build: revert "make image target wait for initramfs"

This reverts commit 43be5087.

The change is incompatible with the image builder code.
Luckily the RT-AC58U is no longer depending on the initramfs
being available for the target's image generation rules.

Reported-by: Venitex Aveon <aveon@aenote.com>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>