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/bcm28155_ap_defconfig b/configs/bcm28155_ap_defconfig
index d26cde5..4f7a58e 100644
--- a/configs/bcm28155_ap_defconfig
+++ b/configs/bcm28155_ap_defconfig
@@ -8,7 +8,7 @@
 # CONFIG_DISPLAY_BOARDINFO is not set
 CONFIG_HUSH_PARSER=y
 # CONFIG_AUTOBOOT is not set
-CONFIG_FASTBOOT=y
+CONFIG_USB_FUNCTION_FASTBOOT=y
 CONFIG_FASTBOOT_BUF_ADDR=0x80000000
 CONFIG_FASTBOOT_BUF_SIZE=0x7FF00000
 CONFIG_FASTBOOT_FLASH=y