scripts/check-config.sh: fix "command not found" error handling

scripts/check-config.sh: fix "command not found" error handling

scripts/check-config.sh exits successfully and silently without doing
any checks when the 'comm' command is not found.

The problem triggers from the command around line 39:

  comm -23 ${suspects} ${ok} >${new_adhoc}

This statement fails when 'comm' is not in $PATH, creating an empty
${new_adhoc} file. But the script continues and the following line,
which is supposed to detect an error:

  if [ -s ${new_adhoc} ]; then

will always be false since the file is empty, and the script will exit
successfully as if everything were OK.

The case where 'comm' in not in $PATH is not theoretical. It used to
happen on yocto until a recent fix [0], and still happens on the
current stable branch (rocko).

Fix by setting the errexit flag to exit with error when a statement
fails, so that at least the problem is noticed.

For additional safety also set the nounset flag to detect expansion
errors.

[0] http://git.yoctoproject.org/cgit/cgit.cgi/poky/commit/?id=fe0b4cb5b48580d4a3f3c0eb82bfa6f1b13801e4

Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Reviewed-by: Simon Glass <sjg@chromium.org>