meta-yocto-bsp: Purge atom-pc in favor of genericx86

meta-yocto-bsp: Purge atom-pc in favor of genericx86

genericx86 is a superset of atom-pc, so remove all references to
atom-pc.

Note that genericx86 only has 3.10 linux-yocto support, that leaves us
with no x86 BSP in meta-yocto-bsp for the 3.4 kernel. As a general rule,
new BSPs are targeted at the current kernel version, so this is normal,
but something worth noting.

Signed-off-by: Darren Hart <dvhart@linux.intel.com>
Cc: Ross Burton <ross.burton@intel.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>