layer_extra_sanity.bbclass: add new bbclass

layer_extra_sanity.bbclass: add new bbclass

Add a new bbclass for extra sanity check for layers.

These sanity checks include:
*) conf/machine and conf/distro don't appear in the same layer.
   The rational is that distro specific changes and BSP specific changes
   should be splitted into different layers for better maintenance.
   The meta layer is an exception.
*) A BSP layer is not included if MACHINE is not set to any of the conf file
   it provides.
   The rational is that BSP layer, by design, is supposed to make changes
   specific for the BSPs it covers. Including a BSP layer which doesn't contain
   a conf file matching the MACHINE setting might cause potential problem.
*) Configuration files under conf/machine don't set distro specific variables
   such as DISTRO_FEATURES.
   The rational is that machine configuration files should not set things
   related to machine.

[YOCTO #5427]

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>