Bug 1458615 - Fix unofficial Android build on systems where $USER is not set. r=nalexander

When `--enable-official-branding` isn't set in mozconfig, `./mach configure` executes
mobile/android/branding/unofficial/configure.sh via `/bin/sh`. The `${USER:-unknown}`
syntax is POSIX, so this fix should work across all platforms.

MozReview-Commit-ID: C8T0ZgOFWgV

--HG--
extra : rebase_source : 32e0025f4a38fcf1113148cd6dbc4506af4a1ece

mobile/android/branding/unofficial/configure.sh

@@ -2,8 +2,8 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-ANDROID_PACKAGE_NAME=org.mozilla.fennec_`echo $USER | sed 's/-/_/g'`
-MOZ_APP_DISPLAYNAME="Fennec `echo $USER | sed 's/-/_/g'`"
+ANDROID_PACKAGE_NAME=org.mozilla.fennec_`echo ${USER:-unknown} | sed 's/-/_/g'`
+MOZ_APP_DISPLAYNAME="Fennec `echo ${USER:-unknown} | sed 's/-/_/g'`"
 MOZ_UPDATER=
 MOZ_ANDROID_ANR_REPORTER=
 MOZ_ANDROID_GCM_SENDERID=965234145045