From help info of "bitbake -S": --dump-signatures=SIGNATURE_HANDLER
                                    [snip] Two common values are none and printdiff but the handler may define more/less[snip]

And the problems are:

1) After checked the code, it seems that we only can add SIGNATURE_HANDLER inside of bitabke, which means that we can't add handler from bbclass or files like meta/lib/oe/sig_handler.py, do we need make it be able to do so ?

2) The name of SIGNATURE_HANDLER might be confused with BB_SIGNATURE_HANDLER, so we may need change another name in the help text.

3) We may need fix "printdiff" handler and make it to replace of bitbake-whatchanged, currently, printdiff is very slow when run something like "bitbake -sprintdiff core-image-minimal", And probably other fixes.