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/am43xx_evm_usbhost_boot_defconfig b/configs/am43xx_evm_usbhost_boot_defconfig
index be99599..2c73dbe 100644
--- a/configs/am43xx_evm_usbhost_boot_defconfig
+++ b/configs/am43xx_evm_usbhost_boot_defconfig
@@ -10,3 +10,4 @@
 CONFIG_SPI_FLASH_MACRONIX=y
 CONFIG_SYS_NS16550=y
 CONFIG_TI_QSPI=y
+CONFIG_OF_LIBFDT=y