usb: Make compiling gadget support optional

There is no need to compile and include this code if it is not used.
CONFIG_USB_GADGET can be used for the purpose.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
diff --git a/configs/omap4_panda_defconfig b/configs/omap4_panda_defconfig
index b66659f..7ee9eea 100644
--- a/configs/omap4_panda_defconfig
+++ b/configs/omap4_panda_defconfig
@@ -36,6 +36,7 @@
 CONFIG_USB_EHCI_HCD=y
 CONFIG_USB_MUSB_UDC=y
 CONFIG_USB_OMAP3=y
+CONFIG_USB_GADGET=y
 CONFIG_USB_HOST_ETHER=y
 CONFIG_USB_ETHER_SMSC95XX=y
 CONFIG_OF_LIBFDT=y