uboot-at91: move BUILD_SUBTARGET from U-Boot/Default to devices

currenlty U-Boot/Default supports only at91 legacy devices.To add
sama5 support, moving BUILD_SUBTARGET from U-Boot/Default to target
devices.

Signed-off-by: Sandeep Sheriker Mallikarjun <sandeepsheriker.mallikarjun@microchip.com>

package/boot/uboot-at91/Makefile

@@ -17,16 +17,17 @@ include $(INCLUDE_DIR)/package.mk
 
 define U-Boot/Default
   BUILD_TARGET:=at91
-  BUILD_SUBTARGET:=legacy
   UBOOT_IMAGE:=u-boot.bin boot.bin
 endef
 
 define U-Boot/at91sam9m10g45ek_nandflash
   NAME:=AT91SAM9M10G45-EK board (NandFlash)
+  BUILD_SUBTARGET:=legacy
 endef
 
 define U-Boot/at91sam9x5ek_nandflash
   NAME:=AT91SAM9X5-EK board (NandFlash)
+  BUILD_SUBTARGET:=legacy
 endef
 
 UBOOT_TARGETS := \