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/P5020DS_NAND_SECURE_BOOT_defconfig b/configs/P5020DS_NAND_SECURE_BOOT_defconfig
index 22492e3..fab89ad 100644
--- a/configs/P5020DS_NAND_SECURE_BOOT_defconfig
+++ b/configs/P5020DS_NAND_SECURE_BOOT_defconfig
@@ -10,3 +10,4 @@
 CONFIG_E1000=y
 CONFIG_SYS_NS16550=y
 CONFIG_FSL_ESPI=y
+CONFIG_OF_LIBFDT=y