yocto-compat-layer: also determine tune flags for each task

yocto-compat-layer: also determine tune flags for each task

locked-sigs.inc groups tasks according to their tune flags (allarch,
i586, etc.). Also retrieve that information while getting signatures,
it will be needed to determine when setting a machine changes tasks
that aren't machine-specific.

Signed-off-by: Patrick Ohly <patrick.ohly@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>