lib/oe/sstatesig.py: dump locked.sigs.inc only when explicitly asked via -S lockedsigs

This was writing out locked-sigs.inc into cwd with every
'bitbake -S' invocation. When the intent is only to to get task
stamps (-S none), or print the difference between them (-S printdiff),
the file is unnecessary clutter.

A couple of selftests/scripts were however relying on this, so they're
adjusted to explicitly request the file.

eSDK code calls dump_lockedsigs() separately via
oe.copy_buildsystem.generate_locked_sigs() and so isn't affected.

(From OE-Core rev: ad57c3cac2a8d3e60222e3cca0685f582dcea135)

Signed-off-by: Alexander Kanavin <alex@linutronix.de>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>

scripts/lib/checklayer/__init__.py

@@ -307,7 +307,7 @@ def get_signatures(builddir, failsafe=False, machine=None, extravars=None):
     cmd += 'bitbake '
     if failsafe:
         cmd += '-k '
-    cmd += '-S none world'
+    cmd += '-S lockedsigs world'
     sigs_file = os.path.join(builddir, 'locked-sigs.inc')
     if os.path.exists(sigs_file):
         os.unlink(sigs_file)