package: add a PRE_PATCH_HOOKS hook point

This will be useful to integrate Xenomai and RTAI into Buildroot, so
that they can patch the kernel before the normal patching process
starts.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>