build: add defaults for PKG_SOURCE, PKG_SOURCE_SUBDIR, PKG_VERSION

This makes it easier to unify versioning of git based package downloads.
PKG_SOURCE_DATE along with an 8-character abbreviation of the git hash
is used as PKG_VERSION, PKG_RELEASE should be used like normal packages.

Signed-off-by: Felix Fietkau <nbd@nbd.name>