local.conf.sample: add genericx86 reference

(From meta-yocto rev: 61c441c4411a9d07c0e669270deb3e92e0fa58f8)

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

meta-yocto/conf/local.conf.sample

@@ -44,6 +44,7 @@
 #
 #MACHINE ?= "atom-pc"
 #MACHINE ?= "beagleboard"
+#MACHINE ?= "genericx86"
 #MACHINE ?= "mpc8315e-rdb"
 #MACHINE ?= "routerstationpro"
 #