board: STiH410-B2260: Add pxefile_addr_r variable

Reading doc/README.distro , we see platform needs to set
pxefile_addr_r to support distro boot.

Signed-off-by: Riku Voipio <riku.voipio@linaro.org>

include/configs/stih410-b2260.h

@@ -31,6 +31,7 @@
 			"fdtfile=stih410-b2260.dtb\0"		\
 			"fdt_addr_r=0x47000000\0"		\
 			"scriptaddr=0x50000000\0"		\
+			"pxefile_addr_r=0x50100000\0"		\
 			"fdt_high=0xffffffffffffffff\0"		\
 			"initrd_high=0xffffffffffffffff\0"	\
 			"ramdisk_addr_r=0x48000000\0"		\