linux: support a space-separated list of patches

The kernel being a component that often needs a fairly important set
of changes to be adapted to a particular hardware platform, having
maximum flexibility on the patching process is a nice
thing. Therefore, as per the discussions from the Buildroot Developer
Day, we add a mechanism to apply a list of patches (that could come
either from URLs, local files or local directories).

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