usb: add (move) CONFIG_USB_HOST to Kconfig

usb: add (move) CONFIG_USB_HOST to Kconfig

The meaning of CONFIG_USB in U-Boot is different from that in Linux.

As you see in drivers/usb/Kconfig of Linux, CONFIG_USB enables the
USB host controller support, while CONFIG_USB_SUPPORT is used to
enable the whole of the USB sub-system.

When I added CONFIG_USB into Kconfig by commit 6e7e9294d321 ("usb:
add basic USB configs in Kconfig"), I planned to follow the Linux's
convention, i.e. CONFIG_USB to enable/disable the USB host support.

Then, commit 68f7c5db2d1e ("usb: Generic USB Kconfig option, that
fits both host and gadget and comments") changed the logic of the
CONFIG_USB to point to the whole of the USB sub-system.  As a result,
currently we do not have an option for USB host.

This commit adds CONFIG_USB_HOST, which will be useful to compile
in the USB host support code.

CONFIG_USB_HOST is not referenced at all, but strangely some boards
define it in board headers.  I removed them because USB_HOST will be
selected in Kconfig going forward.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>