paycheck: Small improvements to the block tracer utility.

This prepends the current block number to the output, simplifies some
logic, and tightens argument validation in the command-line parser.

BUG=None
TEST=paycheck -B/-b works.

Change-Id: I90d5cdf721612cdd12e49f4e4181849fc699807f
Reviewed-on: https://chromium-review.googlesource.com/286547
Tested-by: Gilad Arnold <garnold@chromium.org>
Commit-Queue: Gilad Arnold <garnold@chromium.org>
Reviewed-by: Don Garrett <dgarrett@chromium.org>
diff --git a/scripts/paycheck.py b/scripts/paycheck.py
index 5290e9d..0195f53 100755
--- a/scripts/paycheck.py
+++ b/scripts/paycheck.py
@@ -117,7 +117,7 @@
         parser.error('invalid argument to --disabled_tests: %s' % test)
 
   # Ensure consistent use of block tracing options.
-  do_block_trace = opts.root_block or opts.kern_block
+  do_block_trace = not (opts.root_block is None and opts.kern_block is None)
   if opts.skip and not do_block_trace:
     parser.error('--skip must be used with either --root-block or --kern-block')