image.bbclass: add ability to set systemd default target
Add ability to set the default target for systemd images.
The default target for system is controlled by SYSTEMD_DEFAULT_TARGET.
The default value for this variable is derived from checking whether
IMAGE_FEATURES contains 'x11-base' or not. Each image could override
this value in its own recipe. For now, we don't need to do any change,
because all images that support graphical environment has 'x11-base'
in its IMAGE_FEATURES.
