Update sigul macro to support sign mode

Sign mode is a parameter to allow the user to choose whether or not
to sign artifacts in parallel using gnu-parallel or in serial mode
(bash for loop) methods.

Cleanup obsolete lftools_activate method.

Change-Id: Ib202f93a37e2830e5048c5a766c17d75233c6423
Signed-off-by: Thanh Ha <thanh.ha@linuxfoundation.org>