There is no particular reason why the USB Kconfig option should be specific to
host mode. In prevision of adding MUSB host and gadget to Kconfig, this moves
the title and help message of the USB Kconfig option to a more generic format.

Adding comments to the usb Kconfig allows for a better separation and more
readability in generated configs and in menuconfig.

Signed-off-by: Paul Kocialkowski <contact@paulk.fr>