configs: Odroid C2: enable device tree overlays

The Odroid C2 comes with several expansion options such as an RTC module.
As these expansions are not considered in the Linux device tree they have
to be enabled via device tree overlays.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>

configs/odroid-c2_defconfig

@@ -28,3 +28,4 @@ CONFIG_DEBUG_UART_CLOCK=24000000
 CONFIG_DEBUG_UART_ANNOUNCE=y
 CONFIG_DEBUG_UART_SKIP_INIT=y
 CONFIG_MESON_SERIAL=y
+CONFIG_OF_LIBFDT_OVERLAY=y