initramfs-{debug,kexecboot}-image: zero out the rootfs_extra_space in initramfs images

Setting IMAGE_ROOTFS_EXTRA_SPACE impacts the initramfs images, results
in an error about INITRAMFS_MAXSIZE.

Signed-off-by: Stephano Cetola <stephano.cetola@linux.intel.com>
Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Armin Kuster <akuster808@gmail.com>