build: add mkrevision.sh helper script which properly infers the revision from either svn, git-svn or git work copies

git-svn-id: http://svn.luci.subsignal.org/luci/trunk@9770 ab181a69-ba2e-0410-a84d-ff88ab4c47bc
contrib: use mkrevision.sh to create package version, assume full checkout/clone of LuCI and abort with error for old style feed urls

git-svn-id: http://svn.luci.subsignal.org/luci/trunk@9771 ab181a69-ba2e-0410-a84d-ff88ab4c47bc