bitbake: bitbake-diffsigs: fix handling when finding only a single sigfile

This fixes the following error when calling 'bitbake-dumpsig' or
'bitbake-diffsigs' when having only a single sigfile available:

| Traceback (most recent call last):
|   File "[..]/poky/bitbake/bin/bitbake-dumpsig", line 171, in <module>
|     files = find_siginfo_task(tinfoil, options.taskargs[0], options.taskargs[1])
|             ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
|   File "[..]/poky/bitbake/bin/bitbake-dumpsig", line 83, in find_siginfo_task
|     sig2 = latestsigs[1]
|            ~~~~~~~~~~^^^
| IndexError: list index out of range

Handle this by adding (and returning) the path for the second sigfile
only if one is found. This way it will work for both diffsigs and
dumpsig use case.

The calling argparse code already deals with find_siginfo_task()
returning only a single file.
For 'bitbake-dumpsig' it will just dump the single sigfile, for
'bitbake-diffsigs' it will emit a proper error message again:

| ERROR: Only one matching sigdata file found for the specified task (systemd configure)

(cherry picked from commit 25057d33e9131f3214a06bbb316c916c744f8f03)

(Bitbake rev: 4e443aeab9096b41c9e5ba41cd21027ecaa20285)

Signed-off-by: Enrico Jörns <ejo@pengutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Patrick Vogelaar <patrick.vogelaar@belden.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
Signed-off-by: Steve Sakoman <steve@sakoman.com>

bitbake/bin/bitbake-diffsigs

@@ -72,16 +72,17 @@ def find_siginfo_task(bbhandler, pn, taskname, sig1=None, sig2=None):
         elif sig2 not in sigfiles:
             logger.error('No sigdata files found matching %s %s with signature %s' % (pn, taskname, sig2))
             sys.exit(1)
+
+        latestfiles = [sigfiles[sig1]['path'], sigfiles[sig2]['path']]
     else:
         sigfiles = find_siginfo(bbhandler, pn, taskname)
         latestsigs = sorted(sigfiles.keys(), key=lambda h: sigfiles[h]['time'])[-2:]
         if not latestsigs:
             logger.error('No sigdata files found matching %s %s' % (pn, taskname))
             sys.exit(1)
-        sig1 = latestsigs[0]
-        sig2 = latestsigs[1]
-
-    latestfiles = [sigfiles[sig1]['path'], sigfiles[sig2]['path']]
+        latestfiles = [sigfiles[latestsigs[0]]['path']]
+        if len(latestsigs) > 1:
+            latestfiles.append(sigfiles[latestsigs[1]]['path'])
 
     return latestfiles