Fix the multi- target targets (#5489) (#5491)

On second thought, using % just isn't correct. When a make
target looks like:

foo% bar%:
	...

Then that implies that it will build BOTH foo% and bar% whenever
that rule is triggered (by either) for any value of the pattern %.

Clearly that is not what we want / intend.

So, this commit goes back to using config targets without a % in it
by generating a full list of them.

It also turns sitl_deprecation into a Hidden Target.

Finally, now that we have those target lists anyway, I added
a 'help' and 'list_config_targets' targets that print the
available targets.

Note that the '%' catch-all target only works when all other targets
either have a recipe or are marked as .PHONY, otherwise such targets
are only interpretted as dependencies and still executre the '%'
target afterwards, which is not what we want.