README.hardware: declare support for BeagleBoard xM rev B

The BeagleBoard xM revision B has been tested (by me, if nobody else.)

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>

README.hardware

@@ -142,7 +142,7 @@ faster CPU, more RAM, an ethernet port, more USB ports, microSD, and removes
 the NAND flash. The beagleboard MACHINE is tested on the following platforms:
 
   o Beagleboard C4
-  o Beagleboard xM Rev A
+  o Beagleboard xM rev A & B
 
 The Beagleboard C4 has NAND, while the xM does not. For the sake of simplicity,
 these instructions assume you have erased the NAND on the C4 so its boot