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/drivers/Kconfig b/drivers/Kconfig
index c2e813f..8424898 100644
--- a/drivers/Kconfig
+++ b/drivers/Kconfig
@@ -28,6 +28,8 @@
 
 source "drivers/dma/Kconfig"
 
+source "drivers/fastboot/Kconfig"
+
 source "drivers/firmware/Kconfig"
 
 source "drivers/fpga/Kconfig"