utils/brmake: print the error code of the build

utils/brmake: print the error code of the build

Since the stdout and stderr streams are redirected, it is not
immediately obvious when a build failed, even though brmake really exits
with the same error code as make did.

When there is an error, print the exit code after the elapsed time.

Reported-by: Arnout Vandecappelle <arnout@mind.be>
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>