bitbake-diffsigs: consider the situation where sigdata and siginfo files having the...

bitbake-diffsigs: consider the situation where sigdata and siginfo files having the same hash values

For now, `bitbake-diffsigs -t <recipe> <task>' doesn't work. It always outputs
nothing.

The problem is that bitbake-diffsigs are comparing sigdata and siginfo files
that have the same hash value. This is not what we want. These two files are
actually duplicates considering the purpose of bitbake-diffsigs. So we need
to remove one of them so that bitbake-diffsigs could actually compare the
correct signature files.

Signed-off-by: Chen Qi <Qi.Chen@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>