Convert CONFIG_SPL_USB_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

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 1809776..5bc4ef8 100644
--- a/configs/am43xx_evm_usbhost_boot_defconfig
+++ b/configs/am43xx_evm_usbhost_boot_defconfig
@@ -12,6 +12,7 @@
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_USB_HOST_SUPPORT=y
+CONFIG_SPL_USB_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set