script: Remove systemd directories only if not required.

Mask systemd directories only if the USE variable doesn't list
the "systemd" flag.

BUG=chromium:583671
TEST=Built packages. Checked directories.

Change-Id: I9981a5f03476979279a5aa191d8808dcc0395ddc
Reviewed-on: https://chromium-review.googlesource.com/412642
Commit-Ready: Sabin Flo <sabin.floares@gmail.com>
Tested-by: Sabin Floares <sabin.floares@intel.com>
Tested-by: Sabin Flo <sabin.floares@gmail.com>
Reviewed-by: Andrey Ulanov <andreyu@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
2 files changed