lib: rename last_line to checkpoint

[xiulipan]

What we need is to set a checkpoint for kernel log to let
us start check kernel. Rename last_line to a more generic
name checkpoint.

Signed-off-by: Pan Xiuli xiuli.pan@linux.intel.com

[xiulipan]

@marc-hb Done, please have a look or blind approval.

[xiulipan]

@marc-hb Ping

[marc-hb]

Thanks, the change is good and the name "checkpoint" sounds great but the commit message is a bit incomplete: It's not obvious why sof-kernel-log-check.sh $VAR is the same as sof-kernel-log-check.sh "$VAR" and the commit message should mention why they have the same effect in this particular codebase.

sof-kernel-log-check.sh $VAR is the same as sof-kernel-log-check.sh "$VAR" only because sof-kernel-log-check.sh starts with ${1:-1}.

If sof-kernel-log-check.sh were for instance starting with ${1-1} then sof-kernel-log-check.sh $VAR is the same as sof-kernel-log-check.sh "$VAR" would be a real functional change.

In general one cannot assume that $VAR is the same as "$VAR"`. This difference is not a purely theoretical issue, see real example in #430 (review) . Shellcheck warnings are rarely just about "codestyle" , most shellcheck warnings are about functional changes.