genericx86: Create a genericx86-common.inc base for the x86 BSPs

genericx86: Create a genericx86-common.inc base for the x86 BSPs

The genericx86 and genericx86-64 machines share a great deal in common
in terms of machine features, required packages, etc. Use a common
include file to simplify changes to both machine definitions and avoid
accidental omissions.

Replace the hard-coded XSERVER assignment with the XSERVER_IA32*
defines from ia32-base.inc.

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