usb: gadget: Add and use matching SPL USB ethernet gadget Kconfig symbols

Define SPL_USB_ETH_RNDIS symbol to make it possible to select USB
gadget ethernet support in SPL and U-Boot separately in Kconfig .
Make use of the new symbols in gadget Makefile and move the rndis.o
just below the now merged USB_ETHER symbol in Makefile.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>