tools: kwboot: Cleanup bootmsg and debugmsg variables

tools: kwboot: Cleanup bootmsg and debugmsg variables

Function kwboot_debugmsg() is always called with kwboot_msg_debug as msg
and function kwboot_bootmsg() with kwboot_msg_debug as msg. Function
kwboot_bootmsg() is never called with NULL msg.

Simplify, cleanup and remove dead code.

No functional change.

Signed-off-by: Pali Rohár <[email protected]>
Reviewed-by: Stefan Roese <[email protected]>
Tested-by: Stefan Roese <[email protected]>