toolkit: Prevent license agreement prompt.

The chromeos-base/chromeos-factory ebuild has been changed to fill right
license text (BSD-Google). However, makeself will use LICENSE and
request for agreement on installation, without any option to skip that.

For factory installer since people have either downloaded from CPFE,
buildbot or built from source tree, they have already accepted the
license so we don't need to prompt again.

BUG=none
TEST=emerge-gale chromeos-factory;
     /build/gale/usr/local/factory/bundle/toolkit/install*.run

Change-Id: Idaf17c279d24eb61086ab733f5ef5d887705b5ff
Reviewed-on: https://chromium-review.googlesource.com/409791
Commit-Ready: Hung-Te Lin <hungte@chromium.org>
Tested-by: Hung-Te Lin <hungte@chromium.org>
Reviewed-by: Mao Huang <littlecvr@chromium.org>
Reviewed-by: Pi-Hsun Shih <pihsun@chromium.org>
diff --git a/py/toolkit/installer.py b/py/toolkit/installer.py
index 72a2475..27eb251 100755
--- a/py/toolkit/installer.py
+++ b/py/toolkit/installer.py
@@ -576,4 +576,9 @@
       installer.Install()
 
 if __name__ == '__main__':
+  # makself inteprets "LICENSE" environment variable string as license text and
+  # will prompt user to accept before installation. For factory toolkit, we
+  # don't want any user interaction in installation and the license is already
+  # covered by ebuild or download platform like CPFE.
+  os.putenv('LICENSE', '')
   main()