cros_bundle_firmware: Use 'with' to open image file
cros_bundle_firmware: Use 'with' to open image file

This is more pythonic.

BUG=chrome-os-partner:10075
TEST=emerge-tegra2_seaboard chromeos-bootimage

Change-Id: I8aeca2f6dbe0eaa04ee834f2bc15397213368c46
Reviewed-on: https://gerrit.chromium.org/gerrit/24761
Reviewed-by: Randall Spangler <62698fdbb84d1779579ee80c3f39fac22017e5bc@chromium.org>
Commit-Ready: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>
Tested-by: Simon Glass <c00b0378376498bd9cd974c388df8854c0131d27@chromium.org>

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