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/drivers/usb/gadget/Makefile b/drivers/usb/gadget/Makefile
index 46d7d94..4e15323 100644
--- a/drivers/usb/gadget/Makefile
+++ b/drivers/usb/gadget/Makefile
@@ -19,7 +19,7 @@
 obj-$(CONFIG_USB_FUNCTION_THOR) += f_thor.o
 obj-$(CONFIG_USB_FUNCTION_DFU) += f_dfu.o
 obj-$(CONFIG_USB_FUNCTION_MASS_STORAGE) += f_mass_storage.o
-obj-$(CONFIG_CMD_FASTBOOT) += f_fastboot.o
+obj-$(CONFIG_USB_FUNCTION_FASTBOOT) += f_fastboot.o
 endif
 ifdef CONFIG_USB_ETHER
 obj-y += ether.o