bitbake: bitbake-diffsigs: Fix runtime error when no arguments are given

bitbake: bitbake-diffsigs: Fix runtime error when no arguments are given

If bitbake-diffsigs is run without arguments it will error:

    Traceback (most recent call last):
      File "./bin/bitbake-diffsigs", line 121, in <module>
        if output:
    NameError: name 'output' is not defined

Fix this by moving the check for output into the inner else-clause.

(Bitbake rev: 7d3545a66863ad7183a7650b2af57eee091c45ae)

Signed-off-by: Jacob Kroon <jacob.kroon@mikrodidakt.se>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>