usb: Fastboot function config for better consistency with other functions

USB download gadget functions such as thor and dfu have a separate config option
for the USB gadget part of the code, independent from the command part.
This switches the fastboot USB gadget to the same scheme, for better
consistency.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>
Tested-by: Lukasz Majewski <l.majewski@samsung.com>

Test HW: Odroid_XU3 (Exynos5422), trats (Exynos4210)
diff --git a/arch/arm/cpu/armv7/omap-common/boot-common.c b/arch/arm/cpu/armv7/omap-common/boot-common.c
index bbc6bed..7fc0a56 100644
--- a/arch/arm/cpu/armv7/omap-common/boot-common.c
+++ b/arch/arm/cpu/armv7/omap-common/boot-common.c
@@ -163,7 +163,7 @@
 }
 #endif
 
-#if defined(CONFIG_CMD_FASTBOOT) && !defined(CONFIG_ENV_IS_NOWHERE)
+#if defined(CONFIG_USB_FUNCTION_FASTBOOT) && !defined(CONFIG_ENV_IS_NOWHERE)
 int fb_set_reboot_flag(void)
 {
 	printf("Setting reboot to fastboot flag ...\n");