This renames USB to USB_HOST since this is what the symbol really
means. Introduce a USB symbol which is selected by both USB_GADGET
and USB_HOST. This gives us a symbol to let common USB code depend
on.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Tested on the phyCORE-imx27 to boot on NOR and NAND using the
registered update handlers.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

Conflicts:
	arch/arm/configs/tx25stk5_defconfig

This creates a imx_defconfig which in future can catch all
non thumb i.MX boards not covered by imx_v7_defconfig. Right
now the Karo TX25 and Phytec phyCARD i.MX27 can so multi image
support. They are enabled in the new config and the previous
board specific config is removed.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>