linux: set a few more initramfs opts for newer kernels

Building with linux-2.6.36 and initramfs support causes the build to
pause while it prompts for newer options (uid/gid/compression).  So
have the build system inject the newer options into the linux config
automatically.  Older versions should just ignore these.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>