kdb: use bool for binary state indicators

defcmd_in_progress  is the state trace for command group processing
- within a command group or not -  usable  is an indicator if a command
set is valid (allocated/non-empty) - so use a bool for those binary
indication here.

Signed-off-by: Nicholas Mc Guire <hofrat@osadl.org>
Reviewed-by: Daniel Thompson <daniel.thompson@linaro.org>
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>