at91bootstrap: Add BUILD_SUBTARGET variable

Added sama5 to BUILD_SUBTARGET variable.This will populate at91bootstrap
menu options in bootloader menu only when SAMA5 devices are selected as
SUBTARGET and to avoid showing up this menu when legacy device is
selected as SUBTARGET and fixed typo mistake: sama5d3 -> sama5d2.

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

package/boot/at91bootstrap/Makefile

@@ -24,13 +24,14 @@ include $(INCLUDE_DIR)/package.mk
 
 define AT91Bootstrap/Default
   BUILD_TARGET:=at91
+  BUILD_SUBTARGET:=sama5
   AT91BOOTSTRAP_IMAGE:=at91bootstrap.bin
 endef
 
 define at91bootstrap/sama5d2_xplaineddf_uboot
-  TITLE:=AT91Bootstrap for the SAMA5D3 Xplained board (SPI Flash)
+  TITLE:=AT91Bootstrap for the SAMA5D2 Xplained board (SPI Flash)
   BUILD_SUBTARGET:=sama5
-  BUILD_DEVICES:=at91-sama5d3_xplained
+  BUILD_DEVICES:=at91-sama5d2_xplained
 endef
 
 define at91bootstrap/sama5d2_xplaineddf_qspi_uboot