[Buildroot] [PATCH] Fix issue with printvars executing giant shell command

[Trent Piepho <tpiepho@impinj.com>]

On Mon, 2018-09-10 at 20:18 +0200, Yann E. MORIN wrote:
> On 2018-09-10 17:45 +0000, Trent Piepho spake thusly:
> > On Wed, 2018-08-29 at 22:49 +0200, Yann E. MORIN wrote:
> > > On 2018-08-17 16:15 -0700, Trent Piepho spake thusly:
> > > > 
> > > > The solution here is to use $(strip $(foreach ...)), so the command
> > > > expands to "@:", which make is smart enough to not even execute and
> > > > wouldn't exceed any limits if it did.
> > > > 
> So what about:
> 
> diff --git a/Makefile b/Makefile
> index 2c6af12989..57e776056c 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -988,7 +988,8 @@ $(BUILD_DIR)/.br2-external.in: $(BUILD_DIR)
>  # displayed.
>  .PHONY: printvars
>  printvars:
> -	@:$(foreach V, \
> +	@:
> +	$(foreach V, \
>  		$(sort $(if $(VARS),$(filter $(VARS),$(.VARIABLES)),$(.VARIABLES))), \
>  		$(if $(filter-out environment% default automatic, \
>  				$(origin $V)), \
> 
> In a recipe, a command line that contains only spaces will not be executed
> by make.
> 
> So with the above, you get the silence from a rule, and no long-shell
> commands that break.
> 
> > However, it is clearly defined that strip will convert a sequence of
> > whitespace to an empty string, so I figured better to relay on that.
> 
> I'm just trying to keep it simple.

Ok, if you like that better.  My thought was "strip eats whitespace" is
simple, but "a recipe line starting with a tab and containing only
whitespace is skipped" is more obscure.  In fact, while it does appear
to be true, I haven't found anything in the make docs clearly stating
this.