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/a4m072_defconfig b/configs/a4m072_defconfig
index 4a02293..379b0db 100644
--- a/configs/a4m072_defconfig
+++ b/configs/a4m072_defconfig
@@ -5,3 +5,4 @@
 CONFIG_AUTOBOOT_PROMPT="autoboot in %d seconds\n"
 CONFIG_AUTOBOOT_DELAY_STR="asdfg"
 # CONFIG_CMD_SETEXPR is not set
+CONFIG_OF_LIBFDT=y