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 6e7e9294d3 ("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 68f7c5db2d ("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>

drivers/usb/host/Kconfig

@@ -3,8 +3,12 @@
 #
 comment "USB Host Controller Drivers"
 
+config USB_HOST
+	bool
+
 config USB_XHCI_HCD
 	bool "xHCI HCD (USB 3.0) support"
+	select USB_HOST
 	---help---
 	  The eXtensible Host Controller Interface (xHCI) is standard for USB 3.0
 	  "SuperSpeed" host controller hardware.
@@ -28,6 +32,7 @@ endif # USB_XHCI_HCD
 
 config USB_EHCI_HCD
 	bool "EHCI HCD (USB 2.0) support"
+	select USB_HOST
 	---help---
 	  The Enhanced Host Controller Interface (EHCI) is standard for USB 2.0
 	  "high speed" (480 Mbit/sec, 60 Mbyte/sec) host controller hardware.
@@ -128,6 +133,7 @@ config USB_OHCI_GENERIC
 	bool "Support for generic OHCI USB controller"
 	depends on OF_CONTROL
 	depends on DM_USB
+	select USB_HOST
 	---help---
 	  Enables support for generic OHCI controller.
 
@@ -135,6 +141,7 @@ endif # USB_OHCI_HCD
 
 config USB_UHCI_HCD
 	bool "UHCI HCD (most Intel and VIA) support"
+	select USB_HOST
 	---help---
 	  The Universal Host Controller Interface is a standard by Intel for
 	  accessing the USB hardware in the PC (which is also called the USB