fastboot: Move fastboot to drivers/fastboot

Separate CMD_FASTBOOT from FASTBOOT and move code and configuration to
drivers/fastboot.

Switch dependencies on FASTBOOT to USB_FUNCTION_FASTBOOT as anyone who wants
FASTBOOT before this series wants USB_FUNCTION_FASTBOOT. Split
USB_FUNCTION_FASTBOOT from FASTBOOT so they retain their existing
behaviour.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index d532c9f..9848c06 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -137,8 +137,6 @@
 
 endmenu
 
-source "cmd/fastboot/Kconfig"
-
 config BUILD_BIN2C
 	bool
 
@@ -650,6 +648,18 @@
 	  can be useful to see the state of driver model for debugging or
 	  interest.
 
+config CMD_FASTBOOT
+	bool "fastboot - Android fastboot support"
+	depends on FASTBOOT
+	help
+	  This enables the command "fastboot" which enables the Android
+	  fastboot mode for the platform. Fastboot is a protocol for
+	  downloading images, flashing and device control used on
+	  Android devices. Fastboot requires support for acting as a USB
+	  device.
+
+	  See doc/README.android-fastboot for more information.
+
 config CMD_FDC
 	bool "fdcboot - Boot from floppy device"
 	help