fastboot: Add CONFIG_FASTBOOT_USB_DEV option

Some boards (like AM57x EVM) has USB OTG controller other than 0. So in
order to use correct controller number in compiled environment we should
define CONFIG_FASTBOOT_USB_DEV option.

For example, when doing "fastboot reboot-bootloader" we want to enter
fastboot mode automatically. But to do so we need to provide controller
number to "fastboot" command. If this procedure is defined in some config
which is common to bunch of boards, and boards have different USB
controller numbers, we can't just hardcode "fastboot 0" in the
environment. We need to use configurable option, which this patch adds.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>

cmd/fastboot/Kconfig

@@ -41,6 +41,14 @@ config FASTBOOT_BUF_SIZE
 	  downloads. This buffer should be as large as possible for a
 	  platform. Define this to the size available RAM for fastboot.
 
+config FASTBOOT_USB_DEV
+	int "USB controller number"
+	default 0
+	help
+	  Some boards have USB OTG controller other than 0. Define this
+	  option so it can be used in compiled environment (e.g. in
+	  CONFIG_BOOTCOMMAND).
+
 config FASTBOOT_FLASH
 	bool "Enable FASTBOOT FLASH command"
 	help