kbuild: initramfs cleanup, set target from Kconfig

Rather than keep a list of all possible compression types in the
Makefile, set the target explicitly from Kconfig.

Reviewed-by: Francisco Blas Izquierdo Riera (klondike) <klondike@klondike.es>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>