Dev Install: Add support for --yes option to assume y's to all prompts.

Also some fixes to gmerge workflow to allow easy integration testing.

BUG=chromium-os:12388, chromium-os:37901
TEST=Ran it with devmode-test

Change-Id: If511a65fb81fb2ed3d425005490efc9353283274
Reviewed-on: https://gerrit.chromium.org/gerrit/44763
Commit-Queue: Chris Sosa <sosa@chromium.org>
Reviewed-by: Chris Sosa <sosa@chromium.org>
Tested-by: Chris Sosa <sosa@chromium.org>
diff --git a/gmerge b/gmerge
index 2a321d7..8df098d 100755
--- a/gmerge
+++ b/gmerge
@@ -43,8 +43,13 @@
     rc = subprocess.call(['mount', '-o', 'remount,rw', '/'])
     if rc == 0:
       return
-    answer = raw_input(
-        'Could not mount / as writable.  Install into /usr/local? (Y/n)')
+    try:
+      answer = raw_input(
+          'Could not mount / as writable.  Install into /usr/local? (Y/n)')
+    except EOFError:
+      # Received if stdin is piped through /dev/null.
+      answer = None
+
     if answer and answer[0] not in 'Yy':
       sys.exit('Better safe than sorry.')