Compiling current SVN fails with the following error for me:

 cc1: warnings being treated as errors
 buildroot/buildroot/build_powerpc/makedevs/makedevs.c: In function main:
 buildroot/buildroot/build_powerpc/makedevs/makedevs.c:531: error:
 ignoring return value of system, declared with attribute warn_unused_result

This change adds a simple warning if system() fails.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>