systemd-machine-units: add machine specific recipe for BSPs

* some machines like n900/gta02/palmpre are using extra units to
  initialize something (g_ether, cmt, ...), so make it easy to bbappend
  such units
* stuff like INHERIT_append_crespo = " systemd" from BSP doesn't play well with systemd.bbclass

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>