Readonly detection logic should handle ROOT

When emerging a package, portage check for a read only filesystem from / and not
from ROOT.

BUG=chromium:398269
TEST=build an image, cros_vm_test succeed.

Change-Id: I4a35cbed19af52130311fc0fe634153e8164b46c
Reviewed-on: https://chromium-review.googlesource.com/217483
Tested-by: Bertrand Simonnet <bsimonnet@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
1 file changed