Add detection for initramfs of the form *.img.old. For example, Gentoo's
sys-kernel/genkernel installs it as initramfs-*.img and moves any existing
one to initramfs-*.img.old.
Apply the same scheme to initrd-*.img and initrd-*.gz files for consistency.
Signed-off-by: Oskari Pirhonen <xxc3ncoredxx@gmail.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>
