build: allow passing PKG_SOURCE_VERSION from any Makefile

build: allow passing PKG_SOURCE_VERSION from any Makefile

This can be used to build with a custom version of any of your packages.
For packetd, for instance, you could:

  1. in feeds.conf.mfw, redefine our feed:
  'src-git mfw https://github.com/someone/mfw_feeds.git;custom-packetd'

  2. in the custom-packetd branch of mfw_feeds, edit packetd/Makefile,
  change PKG_SOURCE_URL if you want to use a repository other than the
  official untangle one, and right underneath that add
  PKG_SOURCE_VERSION:=some-packetd-branch (this has to happen before
  source'ing version.mk)

  3. build as usual, without passing -v