meta-estsdk-toolchain: use WORKDIR instead of D to hold locked-sigs
do_locked_sigs() uses ${D} to hold a file as input to the sstate staging code.
Generally this isn't a problem as it happens after packaging, but sometimes (if
the recipe is rebuild, perhaps) do_package() will see the file inside ${D} and
fail because there is an uninstalled file.
Solve this by using a directory under ${WORKDIR} so it can't be packaged.
Signed-off-by: Ross Burton <ross.burton@intel.com> Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
