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_nor_defconfig b/configs/am335x_evm_nor_defconfig
index bbde07f..41a5c79 100644
--- a/configs/am335x_evm_nor_defconfig
+++ b/configs/am335x_evm_nor_defconfig
@@ -12,7 +12,7 @@
 CONFIG_SPL_MTD_SUPPORT=y
 CONFIG_SPL_MUSB_NEW_SUPPORT=y
 CONFIG_SPL_OS_BOOT=y
-CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_CMD_SPL=y
 CONFIG_CMD_SPL_NAND_OFS=0x00080000
 CONFIG_CMD_NAND=y