[Buildroot] Build reproducibility

[Jérôme Pouiller]

Hello,

I noticed a problem with reproducibility of build. If I list targets 
(using make show-targets) on Fedora and on Ubuntu, targets are on 
sorted on same orders. Indeed, result of $(wildcard *.mk) are not 
the same on these systems. 

It seems Make does not sort anymore result of wildcard since 3.82: 
http://comments.gmane.org/gmane.comp.gnu.make.bugs/4260.

Following patch should fix problem.