Improvements of support for watchdogs

[edwardalee]

This PR, co-authored by @Benichiwa , is a companion to PR 1730 in lingua-franca, which adds watchdog support for the C target. It also factors out the watchdog support into its own .c and .h files.

[lhstrh]

This is looking pretty good! My comments are minor...

[lhstrh]

Here are some more recommendations regarding conditional compilation:

Prefer to compile out entire functions, rather than portions of functions or portions of expressions. Rather than putting an ifdef in an expression, factor out part or all of the expression into a separate helper function and apply the conditional to that function.

If you have a function or variable which may potentially go unused in a particular configuration, and the compiler would warn about its definition going unused, mark the definition as __maybe_unused rather than wrapping it in a preprocessor conditional. (However, if a function or variable always goes unused, delete it.)

Within code, where possible, use the IS_ENABLED macro to convert a Kconfig symbol into a C boolean expression, and use it in a normal C conditional:

if (IS_ENABLED(CONFIG_SOMETHING)) {
        ...
}

The compiler will constant-fold the conditional away, and include or exclude the block of code just as with an #ifdef, so this will not add any runtime overhead. However, this approach still allows the C compiler to see the code inside the block, and check it for correctness (syntax, types, symbol references, etc). Thus, you still have to use an #ifdef if the code inside the block references symbols that will not exist if the condition is not met.

At the end of any non-trivial #if or #ifdef block (more than a few lines), place a comment after the #endif on the same line, noting the conditional expression used. For instance:

#ifdef CONFIG_SOMETHING
...
#endif /* CONFIG_SOMETHING */

This is from: https://www.kernel.org/doc/html/v4.10/process/coding-style.html.

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