toaster: fix tasks showing as NoExec

toaster: fix tasks showing as NoExec

Tasks without script type information showed by default
as NoExec; this happens for all Prebuild or Covered
tasks, as script type information comes only on TaskStarted
event. Such a default value may drive confusion, as NoExec value
should be reserved for the NoExec-flagged tasks.

This patch adds a new default value named Unknown that will be
used for all tasks that don't have script type information
available.

    [YOCTO #5327]

Signed-off-by: Alexandru DAMIAN <alexandru.damian@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>