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

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.

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