kill-bb: Add it for killing abnormal bitbake processes

kill-bb: Add it for killing abnormal bitbake processes

There might be processes left after Ctr-C, e.g.:
$ rm -f tmp/cache/default-glibc/qemux86/x86_64/
$ bitbake -p

Press 'Ctrl-C' multiple times during parsing, then bitbake processes may not
exit, and the worse is that we can't start bitbake again, we can't always
reproduce this, but sometime. We can only use "ps ux" to find the processes and
kill them one by one. This tool can kill all of them easily.

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>