pkg-generic: implement source-check targets

This commits extends the common package infrastructure with the
$(1)-source-check and $(1)-all-source-check targets.

The $(1)-source-check target simply calls the newly added
SOURCE_CHECK macro on all items to be downloaded.

The $(1)-all-source-check target will depend on the
$(1)-all-source-check targets of all dependent packages and the
$(1)-source-check target of the current package, which allows to do a
recursive source-check in the dependency tree.

[Thomas: move the code around a bit to avoid repeating the
<pkg>_OVERRIDE_SRCDIR condition, as suggested by Yann.]

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>