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/P2020RDB-PC_SDCARD_defconfig b/configs/P2020RDB-PC_SDCARD_defconfig
index 997887d..e9c27db 100644
--- a/configs/P2020RDB-PC_SDCARD_defconfig
+++ b/configs/P2020RDB-PC_SDCARD_defconfig
@@ -9,3 +9,4 @@
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y
 CONFIG_FSL_ESPI=y
+CONFIG_OF_LIBFDT=y