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/sheevaplug_defconfig b/configs/sheevaplug_defconfig
index 9b41460..a51abc7 100644
--- a/configs/sheevaplug_defconfig
+++ b/configs/sheevaplug_defconfig
@@ -10,3 +10,4 @@
 CONFIG_CMD_DHCP=y
 CONFIG_CMD_PING=y
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y