librolegen: Fix incorrect WEBOS_GIT_TAG value

:Release Notes:
Fix WEBOS_GIT_TAG to include the 'submissions/' prefix that its tags
have.

:Detailed Notes:
librolegen repository doesn't have tag '18' only 'submissions/18'
$ git tag -l
submissions/16
submissions/17
submissions/18
versions/2.0.0
versions/2.1.0

It was working correctly until now, just because we were lucky.
From git help ls-remote:
  When <refs>... are specified, only references matching the given
  patterns are displayed.
Hence:
$ git ls-remote git://github.com/openwebos/librolegen 18
  cbedc69733f65cd2f498787a621c014e219d38ab   refs/tags/submissions/18
$ git ls-remote git://github.com/openwebos/librolegen 17
  af5b0e7b514938d5589c89ab9508ad23dce43e98   refs/tags/submissions/17
$ git ls-remote git://github.com/openwebos/librolegen
  9040954a24115b05219e7dd459dcf91ad05cc739   HEAD
  9040954a24115b05219e7dd459dcf91ad05cc739   refs/heads/master
  85524970dba46557d3c9672455a4a88165efe7f1   refs/notes/review
  7aa394eea6bd76618337772894f7615d0ae8e13a   refs/tags/submissions/16
  af5b0e7b514938d5589c89ab9508ad23dce43e98   refs/tags/submissions/17
  cbedc69733f65cd2f498787a621c014e219d38ab   refs/tags/submissions/18
  9040954a24115b05219e7dd459dcf91ad05cc739   refs/tags/submissions/18^{}
  7aa394eea6bd76618337772894f7615d0ae8e13a   refs/tags/versions/2.0.0
  af5b0e7b514938d5589c89ab9508ad23dce43e98   refs/tags/versions/2.1.0

So if someone creates a tag 'foo/18', ls-remote will return both lines
and the build will fail.

I'll submit an improvement to bitbake git fetcher based on this:
http://lists.openembedded.org/pipermail/bitbake-devel/2013-August/003724.html

:Testing Performed:
Only build tested.

:QA Notes:
No change in image.

:Issues Addressed:
[GF-5328]  Improve build performance and allow to build completely from
           PREMIRROR
[GF-12492] Convert from setting PREFERRED_VERSION centrally to setting
           WEBOS_VERSION in each recipe

Open-webOS-DCO-1.0-Signed-off-by: Martin Jansa <martin.jansa@lge.com>
Change-Id: I2203b8c6f5f0846dbef368cc3882c0abe3bd127a
Reviewed-on: https://g2g.palm.com/3055
Reviewed-by: Herb Kuta <herb.kuta@lge.com>
Reviewed-by: Build Verification
Reviewed-by: Susan Montooth <susan.montooth@lge.com>
Tested-by: Susan Montooth <susan.montooth@lge.com>