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/nitrogen6dl2g_defconfig b/configs/nitrogen6dl2g_defconfig
index 6ef4226..cfb2ed8 100644
--- a/configs/nitrogen6dl2g_defconfig
+++ b/configs/nitrogen6dl2g_defconfig
@@ -9,7 +9,7 @@
 CONFIG_SUPPORT_RAW_INITRD=y
 CONFIG_BOARD_EARLY_INIT_F=y
 CONFIG_HUSH_PARSER=y
-CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_ADDR=0x12000000
 CONFIG_CMD_BOOTZ=y
 CONFIG_CMD_MEMTEST=y