scripts: update_gpu_drivers: create a different branch for each update

It is unconvenient to use a single gpudriverbinupdate branch for the
package update, if the branch name was accidentally left over from
a previous update.

Instead, create a separate branch for each package update.

BUG=None
TEST=./update_gpu_drivers.sh --package img-ddk

Change-Id: I0c204338286e6be6b1197e11c923837dfba84482
Reviewed-on: https://chromium-review.googlesource.com/312827
Commit-Ready: Nicolas Boichat <drinkcat@chromium.org>
Tested-by: Nicolas Boichat <drinkcat@chromium.org>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
diff --git a/update_gpu_drivers.sh b/update_gpu_drivers.sh
index fbfd214..58509bc 100755
--- a/update_gpu_drivers.sh
+++ b/update_gpu_drivers.sh
@@ -194,7 +194,7 @@
       cd "${SRC_ROOT}/overlays/${opath}/${category}/${pnbin}"
 
       # following git commands may fail if they have been issued previously
-      git checkout -b gpudriverbinupdate m/master
+      git checkout -b "gpudriverbinupdate-${pv}" m/master
       git mv "${pnbin}-${pvbin}.ebuild" "${pnbin}-${pv}.ebuild"
 
       "ebuild-${oboard}" "${pnbin}-${pv}.ebuild" manifest