Allow for skipping build in the setup script

This can be helpful if i.e. after flashing a user simply wants to
install the application(s) with the right set of permissions.

Also saves time for make to figure out that no changes need to be built.

Bug: n/a
Test: manual
Change-Id: Ib8387389c00be2c9ab1427a9cb12d42ea29a164b