usb:gadget: Disallow DFU in SPL for now

Previously, DFU was not built in for SPL and often disabled via the board
config.h file, in the SPL build.  By moving DFU to Kconfig we now need to
move this logic to the Makefile to continue to allow boards to fit within
their SPL size limit (until gcc 6 is more widespread and unused strings will
be discarded).

Signed-off-by: Tom Rini <trini@konsulko.com>

drivers/usb/gadget/Makefile

@@ -19,7 +19,9 @@ obj-$(CONFIG_USB_GADGET_FOTG210) += fotg210.o
 obj-$(CONFIG_CI_UDC)	+= ci_udc.o
 obj-$(CONFIG_USB_GADGET_DOWNLOAD) += g_dnl.o
 obj-$(CONFIG_USB_FUNCTION_THOR) += f_thor.o
+ifndef CONFIG_SPL_BUILD
 obj-$(CONFIG_USB_FUNCTION_DFU) += f_dfu.o
+endif
 obj-$(CONFIG_USB_FUNCTION_MASS_STORAGE) += f_mass_storage.o
 obj-$(CONFIG_USB_FUNCTION_FASTBOOT) += f_fastboot.o
 endif