Makefile: allow to gzip files.
Makefile: allow to gzip files.

Compress initramfs cpio archive like this:
$ make initramfs.cpio.gz
or initrd:
$ make initrd.cpio.gz

Signed-off-by: Gaël PORTAY <1666f60cfa201a60b0ea694168a0b6932673170a@gmail.com>

gazoo74/initramfs,gazoo74/initramfs