Add option to gmerge to include masked files.
Add option to gmerge to include masked files.

If you're merging a relatively small package (i.e. not chrome) where debug
symbols fit on the device, it's useful to be able to install these
files.

BUG=chromium-os:11914
TEST=Test that cromo debug symbols can be added with include_masked_files,
     and removed in turn by removing that option.
     NOTE: Using this option with Chrome will fill up your disk unless
     you've increased the size of your root partition.

Change-Id: I57e45e560b5bd6505690378b7e2edaa4c66dc7d6
Reviewed-on: http://gerrit.chromium.org/gerrit/1191
Reviewed-by: Chris Sosa <959d3d631fb6f17f1b07f751372041a1edb210d1@chromium.org>
Tested-by: David James <f231dc574bbeb62604660cbc162d1b8642a0a3af@chromium.org>

marineam/coreos-dev-util,coreos/dev-util,mgireesh05/dev-util,marineam/coreos-dev-util,coreos/dev-util,coreos/dev-util,mgireesh05/dev-util,mgireesh05/dev-util