LuCI: Display the proper LuCI git branch in GUI

LuCI: Display the proper LuCI git branch in GUI

Adjust luci.mk and luci-base to find out correctly if Luci is built
from master or from a branch. Display that value in the footer and
the overview page.

* Evaluate the git branch in luci.mk and pass that to LuCI submodules
  as a make variable. Use branch name, ignore tags.
* Deprecate svn detection in luci-base's mkversion.sh that generates
  version.lua. Simply use the value passed by make.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>