This implements customizable initramfs compression, based on the Pentoo
genkernel fork (SVN revs r2142, r2145, by Jens Pranaitis
<jens@chaox.net>).
New configuration option COMPRESS_INITRD_TYPE to allow manual selection,
whereas the Pentoo fork provided auto-selection based on what was
available in the kernel configuration. Auto-selection is used by
default, and is available with the argument of 'best' to the new option.
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>