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/xilinx_zynqmp_zcu106_revA_defconfig b/configs/xilinx_zynqmp_zcu106_revA_defconfig
index a5fa33e..34e3c85 100644
--- a/configs/xilinx_zynqmp_zcu106_revA_defconfig
+++ b/configs/xilinx_zynqmp_zcu106_revA_defconfig
@@ -18,7 +18,7 @@
 CONFIG_SPL_RAM_DEVICE=y
 CONFIG_SPL_ATF=y
 CONFIG_SYS_PROMPT="ZynqMP> "
-CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_FASTBOOT_FLASH=y
 CONFIG_FASTBOOT_FLASH_MMC_DEV=0
 CONFIG_CMD_THOR_DOWNLOAD=y