image-systemd: Support enabling/disabling units at image creation time.

Sometimes it is useful to have a set of units that get enabled or
disabled for an image, but that aren't enabled by default during package
installation.

The image-systemd.bbclass allows for adding a list of whitespace
separated units to the SYSTEMD_UNITS_ENABLE, SYSTEMD_UNITS_DISABLE, or
SYSTEMD_UNITS_MASK variables, in order to run the corresponding systemctl
command only as a rootfs postprocess.

Signed-off-by: Randy Witt <rewitt@declaratino.com>