clean-for-rebuild: also remove images_installed stamp

Without this, a linux-rebuild or bootloader-rebuild will not install
the rebuilt image in the images directory.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>