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/am3517_evm_defconfig b/configs/am3517_evm_defconfig
index c7699e0..9ac43f0 100644
--- a/configs/am3517_evm_defconfig
+++ b/configs/am3517_evm_defconfig
@@ -12,3 +12,4 @@
 # CONFIG_CMD_SETEXPR is not set
 CONFIG_CMD_GPIO=y
 CONFIG_SYS_NS16550=y
+CONFIG_OF_LIBFDT=y