usb: gadget: Move CONFIG_USB_GADGET to Kconfig

The description was borrowed from kernel. "tristate" type was changed
to "bool" (I believe we don't support modules for u-boot yet, right?).
CONFIG_USB_GADGET requires CONFIG_USB to be defined too, so add it along
as well.

Definitions were added to defconfig files in a way that
"make savedefconfig" generates exactly the same file as used defconfig.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
[trini: Add zynq_zc702 conversion]
Signed-off-by: Tom Rini <trini@konsulko.com>
diff --git a/drivers/usb/Kconfig b/drivers/usb/Kconfig
index abb06fc..7fa99c6 100644
--- a/drivers/usb/Kconfig
+++ b/drivers/usb/Kconfig
@@ -88,4 +88,6 @@
 
 endif
 
+source "drivers/usb/gadget/Kconfig"
+
 endif