packagegroup-core-x11: split machine specific parts to separate recipe packagegroup...

packagegroup-core-x11: split machine specific parts to separate recipe packagegroup-core-x11-server

* add new packagegroup-core-x11-server to SIGGEN_EXCLUDERECIPES_ABISAFE,
  so that recipes depending on it are not rebuilt after every machine
  swtich
* allows to remove task-x11-server and task-x11 from meta-oe without
  loosing any functionality
* be carefull with default XSERVER value which does not have
  xf86-input-mouse and xf86-input-keyboard)
* VIRTUAL-RUNTIME_xserver_common which defaults to x11-common in oe-core
  and xserver-common in meta-oe's task-x11

(From OE-Core rev: a398c96706c119f298f57a929a317fcf8e0f5b92)

Signed-off-by: Martin Jansa <Martin.Jansa@gmail.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>