Avoid bash only "[[" and "]]", access variables with curly braces and
quote them.

Checkpatch is not even used in this project.

git-clang-format ensures that all *new* code being submitted adheres to
the format dictated by clang-format.

The code style is based on LLVM (the creator of clang-format), but with
4 instead of 2 spaces indent width and a maximum of 120 instead of 80
characters per line in order to blend in well with existing code.

Please note:
- The er-coap-13 code gets exempt, as at some point, we probably want to
  to interact with whatever upstream there might be. For those files,
  just do what existing code does (which varies every few lines).
- Default is clang-format-10, which is available in the Ubuntu 20.04
  GitHub runner.

I used the following script:

for f in $(find . -type f \( -name '*.c' -or -name '*.h' \) \
    -not -path '*/examples/shared/tinydtls/*' \
    -not -path '*/coap/er-coap-13/*'); do
      end="$(grep --line-number --extended-regexp "\\*{70}/" "${f}" |
        awk -F: '{print $1}')"
      echo "Formatting ${f}, line 1 to ${end}"
      clang-format-10 -i --lines="1:${end}" "${f}"
done

The last commit was purely cosmetic. Therefore, adding it to
.git-blame-ignore-revs.

Doing so gives the developer feedback early on, before the changes hit
the GitHub CI infrastructure.

This would be a good candidate for using a Git pre-hook. Unfortunately,
gitlint is a bit tricky to be used in a pre-push hook:
jorisroovers/gitlint#55