Convert CONFIG_SPL_USB_HOST_SUPPORT to Kconfig

Move this option to Kconfig and tidy up existing uses.

Signed-off-by: Simon Glass <sjg@chromium.org>

configs/am43xx_evm_usbhost_boot_defconfig

@@ -5,12 +5,13 @@ CONFIG_ISW_ENTRY_ADDR=0x40300350
 CONFIG_SPL_STACK_R_ADDR=0x82000000
 CONFIG_DEFAULT_DEVICE_TREE="am437x-gp-evm"
 CONFIG_FIT=y
-CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1,NAND,SPL_USB_HOST_SUPPORT"
+CONFIG_SYS_EXTRA_OPTIONS="SERIAL1,CONS_INDEX=1,NAND"
 CONFIG_SPL_LOAD_FIT=y
 CONFIG_VERSION_VARIABLE=y
 CONFIG_SPL=y
 CONFIG_SPL_STACK_R=y
 CONFIG_SPL_MTD_SUPPORT=y
+CONFIG_SPL_USB_HOST_SUPPORT=y
 CONFIG_HUSH_PARSER=y
 CONFIG_CMD_BOOTZ=y
 # CONFIG_CMD_IMLS is not set