fastboot: Rename fb_set_reboot_flag to fastboot_set_reboot_flag

Rename fb_set_reboot_flag to fastboot_set_reboot_flag so it matches
all other fastboot code in the global name space. Fix the guards around
them so that they're dependent on FASTBOOT, not just USB_FUNCTION_FASTBOOT.

Move the weak implementation of fastboot_set_reboot_flag to fb_common.c
so we can call it from non-USB fastboot code.

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/include/fastboot.h b/include/fastboot.h
index 6cd44d2..816e71b 100644
--- a/include/fastboot.h
+++ b/include/fastboot.h
@@ -41,5 +41,5 @@
  * @response: Pointer to fastboot response buffer
  */
 void fastboot_okay(const char *reason, char *response);
-
+int fastboot_set_reboot_flag(void);
 #endif /* _FASTBOOT_H_ */