Implement --[no-]compress-initramfs option to complement --[no-]integrated-initramfs

Implement --[no-]compress-initramfs option to complement --[no-]integrated-initramfs

When using an integrated initramfs (including in kernel during build),
it's better NOT to compress the initramfs, as the kernel build process
will then be re-compressing it. This can provide space savings on disk
as well as potential time & memory savings (one less decompress pass is
needed before the kernel can use the initramfs).

This functionality was previously available in the undocumented
COMPRESS_INITRD variable.

Code-by: Richard Yao <ryao@cs.stonybrook.edu>
Signed-off-by: Robin H. Johnson <robbat2@gentoo.org>
(commit message rewritten).