If we're building the extensible SDK we don't need to see the "Writing
locked sigs" message; it's only necessary when the user explicitly runs
bitbake -S none <target>.
(From OE-Core rev: 440a351ee13920045c9d3e51882908f7b3f01d35)
Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>