fix(sys-kernel/coreos-kernel): Re-enable gzip (and more) for initramfs

I forgot that we are using gzip'ed cpio images for PXE. Just enable all
the compression methods to avoid similar surprises.
fix(sys-kernel/coreos-kernel): Move AT keyboard back to bzImage

As much as I like not seeing the i8042 error in the kernel log on
platforms without it I foresee someone being really ticked off with me
for making this a module when dracut fails without loading the PS/2 and
keyboard modules making the rescue shell kinda hard to use unless a
serial console is also available.

Yeah, well, fair enough future me. You win.
Merge pull request #257 from marineam/modules

Modules