Bug 1413041 - Don't set LLVM_CONFIG on beta and release. r=gps

LLVM_CONFIG is only allowed when building stylo.  If not building it, it causes invalid option error.

mozconfigs doesn't have same value for milestone.is_nightly of moz.configure.
So, to detect nightly version, I analyze milestone.txt.

MozReview-Commit-ID: Iq1FvxymKEc

--HG--
extra : rebase_source : e07aaf1ee82e7459d97e6558f95967ac7972af9f

mobile/android/config/mozconfigs/common

@@ -8,7 +8,10 @@ if [ "x$IS_NIGHTLY" = "xyes" ]; then
 fi
 
 if [ -z "$NO_NDK" ]; then
-  . "$topsrcdir/build/mozconfig.stylo"
+  # MOZ_UPDATE_CHANNEL isn't always set.  It cannot use to detect as trunk.
+  if [ `tail -1 "$topsrcdir/config/milestone.txt" | grep 'a1'` ] ; then
+    . "$topsrcdir/build/mozconfig.stylo"
+  fi
 fi
 
 MOZ_AUTOMATION_L10N_CHECK=0