Change format of LAYERS to include a URL followed by a branch.

fetch.sh will now clone the url and checkout the branch field.
The loop looks a bit funny on account of accomodating LAYERS files
without a newline before EOF.  The new format is:
$ cat LAYERS
url branch

'branch' can be omitted and master is assumed.
Merge branch 'master' into selinux
Update LAYERS to include the branch 'minimal' for the meta-selinux layer.