usb: gadget: Move CONFIG_USB_GADGET_VBUS_DRAW to Kconfig

The description was borrowed from kernel. Definitions were added to
defconfig files in a way that "make savedefconfig" generates exactly
the same file as used defconfig.

Boards using 0 mA as CONFIG_USB_GADGET_VBUS_DRAW value were moved to use
2 mA (as minimal allowed by Kconfig).

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>