post_gerrit_comment.sh: Add a helper to limit size of Gerrit comment

Trim the comment to be posted to be under 16K, which appears to be
Gerrit's limit (violating it causes error and build failure).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I645b84b5e6d931004f526a3463044af77151e7b5
(cherry picked from commit cf3f73c97f69c7454f155c42f00dd06de255d46d)
2 files changed