checklayer: generate locked-sigs.inc under builddir
yocto-check-layer will find locked-sigs.inc under builddir,
but locked-sigs.inc is generated under current bitbake working
dir. if run yocto-check-layer outside builddir, we will met error
like "No such file or directory: *locked-sigs.inc". change to
run bitbake -S under builddir to fix this problem.
[YOCTO #12973]
Signed-off-by: Changqing Li <changqing.li@windriver.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
