systemd: always depend on the explicit serial console units

systemd: always depend on the explicit serial console units

The systemd-serialgetty recipe generates explicit units for consoles
that are defined in SERIAL_CONSOLES, and if that variable is not defined
then just produces an empty package.

Even when systemd has been configured to use the getty generator, if
there are explicit consoles defined then we should respect them.  Don't
conditionalise the dependency on systemd-serialgetty so that we always
pull in the explicit consoles.

Signed-off-by: Ross Burton <ross.burton@arm.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>