Move CONFIG_OF_LIBFDT to Kconfig

Move this option to Kconfig and tidy up existing boards.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/rpi_defconfig b/configs/rpi_defconfig
index 5d27909..7fc8bbc 100644
--- a/configs/rpi_defconfig
+++ b/configs/rpi_defconfig
@@ -7,3 +7,4 @@
 # CONFIG_CMD_FPGA is not set
 CONFIG_CMD_GPIO=y
 CONFIG_PHYS_TO_BUS=y
+CONFIG_OF_LIBFDT=y