Slight clean up of uart.setup code

[jasaw]

The current code is a little confusing to read and doesn't return error when given an invalid input. There are 2 code paths here:

1. uart port is our console. In this case, we accept an optional echo parameter. If echo is not provided, we leave the existing input_echo flag unchanged.
2. uart port is NOT our console. We accept an optional pins config table parameter.

This PR cleans up the code a bit to make it clearer.

This PR makes changes on top of PR #3540

@jmattsson Please review.

[marcelstoer]

Since you have if (!lua_isnoneornil(L, 6)) { in both the if and the else branch I don't see why this couldn't be the primary condition to check (i.e. guarding the if-else with it).

[jmattsson]

That's a fair point @marcelstoer. It would probably be a bit nicer. I resolved the merge conflict that appeared when I merged the other patches, but I'll hold off merging this one until @jasaw has had a chance to comment.

[jasaw]

@marcelstoer @jmattsson I've moved if (!lua_isnoneornil(L, 6)) { to the outside of the if-else block as suggested.

[jmattsson]

Cheers!