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 <pali@kernel.org>
Reviewed-by: Stefan Roese <sr@denx.de>
Tested-by: Stefan Roese <sr@denx.de>