Add support for patching source packages.

Add support for patching source packages.

Sometimes there's a need to patch upstream source. While this can be
scripted using prebuild commands, it's very verbose and unstructured,
so add dedicated patching support.

Details:

To patch source, specific build's metadata line should include:

patch=patch1.patch;patch2.patch;...

Patches are separated with ';', at least one patch should be specified.
They are applied in the order given. Patches are looked up in
metadata/<package>/ directory. Different builds obviously may reference
the same patch(es).