kbuild: Include gen_initramfs_list.sh and the file list in the .d file

Expand the dependency set used for the initrd to include the
CONFIG_INITRAMFS_SOURCE file and the generator script itself.
Otherwise changing the initramfs file list does not rebuild the CPIO.

Signed-off-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Michal Marek <mmarek@suse.cz>