board/wandboard/post-build.sh: create boot.scr at TARGET_DIR/boot
There is no reason to create boot.scr at board/wandboard and later
install it at TARGET_DIR/boot, leaving a stale file behind.
Signed-off-by: Carlos Santos <unixmania@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>