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/configs/socfpga_sockit_defconfig b/configs/socfpga_sockit_defconfig
index f45c3ed..6a13504 100644
--- a/configs/socfpga_sockit_defconfig
+++ b/configs/socfpga_sockit_defconfig
@@ -26,3 +26,4 @@
 CONFIG_DESIGNWARE_SPI=y
 CONFIG_USB=y
 CONFIG_DM_USB=y
+CONFIG_USB_GADGET=y