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/am335x_evm_usbspl_defconfig b/configs/am335x_evm_usbspl_defconfig
index dc9ac21..4401291 100644
--- a/configs/am335x_evm_usbspl_defconfig
+++ b/configs/am335x_evm_usbspl_defconfig
@@ -19,7 +19,7 @@
 CONFIG_SPL_USB_GADGET_SUPPORT=y
 CONFIG_SPL_USB_ETHER=y
 # CONFIG_SPL_YMODEM_SUPPORT is not set
-CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0x00080000
 # CONFIG_CMD_FLASH is not set