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/rainier_ramboot_defconfig b/configs/rainier_ramboot_defconfig
index 9766835..dcd68fd 100644
--- a/configs/rainier_ramboot_defconfig
+++ b/configs/rainier_ramboot_defconfig
@@ -3,3 +3,4 @@
 CONFIG_TARGET_SEQUOIA=y
 CONFIG_SYS_EXTRA_OPTIONS="RAINIER,SYS_RAMBOOT,SYS_TEXT_BASE=0x01000000,SYS_LDSCRIPT=board/amcc/sequoia/u-boot-ram.lds"
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y