firmware: minor screendump related cleanups

- Remove unused redefinitions of screen_dump() from bootloaders
- Use empty do-while when screendump is compiled out

Change-Id: I3ccdb0390ddaa28d8f561ff744d0db6aaef17f5d

bootloader/mpio_hd200_hd300.c

@@ -539,9 +539,3 @@ void main(void)
     }
 
 }
-
-/* These functions are present in the firmware library, but we reimplement
-   them here because the originals do a lot more than we want */
-void screen_dump(void)
-{
-}