usb: Generic USB Kconfig option, that fits both host and gadget and comments

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>

drivers/usb/Kconfig

-menu USB
-
-config USB
-	bool "Support for Host-side USB"
+menuconfig USB
+	bool "USB support"
 	---help---
 	  Universal Serial Bus (USB) is a specification for a serial bus
 	  subsystem which offers higher speeds and more features than the
@@ -15,8 +13,10 @@ config USB
 	  such as scanners, keyboards, mice, modems, cameras, disks,
 	  flash memory, network links, and printers to the PC.
 
-	  Say Y here if your computer has a host-side USB port and you want
-	  to use USB devices.  You then need to say Y to at least one of the
+	  Say Y here if your device has an USB port, either host, peripheral or
+	  dual-role.
+
+	  For an USB host port, you then need to say Y to at least one of the
 	  Host Controller Driver (HCD) options below.  Choose a USB 1.1
 	  controller, such as "UHCI HCD support" or "OHCI HCD support",
 	  and "EHCI HCD (USB 2.0) support" except for older systems that
@@ -51,6 +51,8 @@ source "drivers/usb/host/Kconfig"
 
 source "drivers/usb/emul/Kconfig"
 
+comment "USB peripherals"
+
 config USB_STORAGE
 	bool "USB Mass Storage support"
 	---help---
@@ -85,4 +87,3 @@ endchoice
 endif
 
 endif
-endmenu