sabotage's linux kernel headers

linux headers are often broken for userspace, so we provide
our own copy for the archs supported by sabotage.
additionally downloading the entire kernel tarball just to
get the headers is overkill.

this currently uses headers of kernel 3.12.6, which is what
Debian is now using. There is a previous 3.3.4 branch.

Installation:
$ make ARCH=xxx prefix=/usr DESTDIR=/opt/package install
where ARCH is the name of one of the arch-specific directories.
for example, i386, x86_64, x32, powerpc, mips, microblaze.

