base/useradd/sstate: Ensure do_package setscene has correct fakeroot dependencies

The do_package_setscene task needs to depend on fakeroot in order to correctly
install its files.

We can whitelist the dependency in the sstate handling code for some
performance improvements since we only need this if we're installing the
package from sstate.

Also use an append operator in base.bbclass for clarity.

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>