fastboot: Extract fastboot_okay/fail to fb_common.c

Add drivers/fastboot/fb_common.c, where fastboot_okay/fail are implemented
so we can call them from a non-USB implementation.

Introduce fastboot_response which takes varargs parameters so we can
use it to generate formatted response strings. Refactor fastboot_okay/fail
to use it.

Signed-off-by: Alex Kiernan <alex.kiernan@gmail.com>
Reviewed-by: Joe Hershberger <joe.hershberger@ni.com>
diff --git a/drivers/fastboot/Makefile b/drivers/fastboot/Makefile
index 651fbf0..b38dcff 100644
--- a/drivers/fastboot/Makefile
+++ b/drivers/fastboot/Makefile
@@ -1,5 +1,7 @@
 # SPDX-License-Identifier:      GPL-2.0+
 
+obj-y += fb_common.o
+
 ifdef CONFIG_FASTBOOT_FLASH_MMC_DEV
 obj-y += fb_mmc.o
 endif