ar71xx: clarify the nand subtarget a bit

SVN-Revision: 21679

target/linux/ar71xx/nand/target.mk

@@ -1,4 +1,4 @@
-BOARDNAME:=Devices with NAND flash
+BOARDNAME:=Devices with NAND flash (mostly Mikrotik)
 
 define Target/Description
 	Build firmware images for Atheros AR71xx/AR913x based boards with