x86, boot: Carve out early cmdline parsing function

Carve out early cmdline parsing function into .../lib/cmdline.c so it
can be used by early code in the kernel proper as well.

Adapted from arch/x86/boot/cmdline.c.

Signed-off-by: Borislav Petkov <bp@suse.de>
Link: http://lkml.kernel.org/r/1400525957-11525-2-git-send-email-bp@alien8.de


Signed-off-by: H. Peter Anvin <hpa@zytor.com>