misc-sanity-checks.sh: Some cleanups

misc-sanity-checks.sh: Some cleanups

Some cleanups for commit 4fdda65a5b9f379af997e1e971865ccfa7a93156.

- Don't assume the path of mktemp command.  It's /usr/bin/mktemp
  for some platforms. eg. NetBSD.  Also, always provide a template
  as it's necessary for some platforms, eg. OS X.  This snippet was
  taken from the example in NetBSD's mktemp(1).
- Move a comment to the appropriate place.
- Improve a regex to ignore more comments.
- As the pattern for find -path is not a regex, no escape is necessary
  for period.

Closes-Bug: #1405584
Change-Id: Ia8358f0f7ebe9bc445ce5aa3c4f340546f37db05