use patched kernel-headers package from sabotage linux

this has several advantages:
- download 700 KB instead of 70MB
- much faster to unpack and "build"
- headers are patched for userspace so you don't get clashes with
  struct ethhdr and similar things.
  this is necessary to build busybox and other low-level linux utils.

only disadvantage:
- does not support new archs out-of-the-box, so when doing a port,
  one has to fallback to the vanilla linux tarball.

to achieve this in a non-ugly way, fetch-extract routine was changed
to allow usage with a single URL. it extracts the required path
components automatically.
LINUX_HEADERS_VERSION was removed as the URL is the only place using it.

closes #20
build.sh: explicitly quit with success when reaching the end

otherwise the $? variable will contain and report the status of the
last executed command.
for example building "sh4" returned error due to this last commmand:
+ [ sh4-linux-musl-strip != sh4-musl-linux-strip -a ! -e sh4-musl-linux-strip ]
make it possible to use binutils tarball which is not from gnu.org (e.g. snapshots)