configs: sama5d3_xplained: add fdt overlay support

Add commands for fdt overlay merging. This is required for the boot scripts
that detect PDAs and apply specific overlays to the DTB passed on to kernel.
Signed-off-by: NEugen Hristev <eugen.hristev@microchip.com>

configs/sama5d3_xplained_mmc_defconfig

@@ -80,3 +80,4 @@ CONFIG_W1=y
 CONFIG_W1_GPIO=y
 CONFIG_W1_EEPROM=y
 CONFIG_W1_EEPROM_DS24XXX=y
+CONFIG_OF_LIBFDT_OVERLAY=y

configs/sama5d3_xplained_nandflash_defconfig

@@ -77,3 +77,4 @@ CONFIG_W1_GPIO=y
 CONFIG_W1_EEPROM=y
 CONFIG_W1_EEPROM_DS24XXX=y
 CONFIG_FAT_WRITE=y
+CONFIG_OF_LIBFDT_OVERLAY=y