configs: sama5d4_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: Eugen Hristev <eugen.hristev@microchip.com>
diff --git a/configs/sama5d4_xplained_spiflash_defconfig b/configs/sama5d4_xplained_spiflash_defconfig
index 13869da..3f519cd 100644
--- a/configs/sama5d4_xplained_spiflash_defconfig
+++ b/configs/sama5d4_xplained_spiflash_defconfig
@@ -88,3 +88,4 @@
 CONFIG_W1_GPIO=y
 CONFIG_W1_EEPROM=y
 CONFIG_W1_EEPROM_DS24XXX=y
+CONFIG_OF_LIBFDT_OVERLAY=y