package: support multiple patches

With this commit, we extend the behaviour of the <pkg>_PATCH variable
so that it now allows to list several patches to be downloaded and
applied, and no longer just one patch.

This will be useful for the elfutils package, and should anyway not
break the existing behaviour for packages using just one patch.

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