[OE-core] Suggestion: sanity check for git branch value

[Carlos Rafael Giani]

Hello,

several recipes have SRC_URI lines like: 
"git://server.name/repository;branch=${SRCBRANCH}". If for some reason 
SRCBRANCH is empty, this essentially evaluates to "branch=". I think a 
simple sanity check that outputs an error if the branch value is empty 
would be useful, since otherwise, this leads to a very confusing error 
later (git fetcher tries to use a command line like: --set-upstream 
origin/  , which causes git to complain that "origin/" is not valid).

thoughts?