sstatesig: fix overrides behaviour to remove SIGGEN_LOCKEDSIGS_i586

sstatesig: fix overrides behaviour to remove SIGGEN_LOCKEDSIGS_i586

Require a sig file which SIGGEN_LOCKEDSIGS_i586 is not null, but
the actual SIGGEN_LOCKEDSIGS_i586 is null.
Invoking 'bitbake -e' and we got:
...
 3935 # $SIGGEN_LOCKEDSIGS_i586 [2 operations]
 3936 #   set /path/to/locked-sigs.inc:8576
 3938 #   del data_smart.py:406 [finalize]
 3939 #     ""
...

It was caused by the following commit:
...
(Bitbake rev: 899d45b90061eb3cf3e71029072eee42cd80930c)
Author: Richard Purdie <richard.purdie@linuxfoundation.org>
Date:   Tue May 31 23:52:50 2011 +0100

    bitbake/data_smart: Change overrides behaviour to remove expanded
      variables from the datastore
...

We add prefix 't-' to type to workaround the overrides behaviour.

(From OE-Core rev: f6a39cc957bf85ff43513f0b76afc3b2c9c906b6)

Signed-off-by: Hongxu Jia <hongxu.jia@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>