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/configs/mx6qsabrelite_defconfig b/configs/mx6qsabrelite_defconfig
index 966e823..a4740d4 100644
--- a/configs/mx6qsabrelite_defconfig
+++ b/configs/mx6qsabrelite_defconfig
@@ -9,7 +9,7 @@
 # CONFIG_USE_BOOTCOMMAND is not set
 CONFIG_SYS_CONSOLE_OVERWRITE_ROUTINE=y
 CONFIG_BOARD_EARLY_INIT_F=y
-CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_ADDR=0x12000000
 CONFIG_CMD_MEMTEST=y
 CONFIG_SYS_ALT_MEMTEST=y