Add brmake, a small script that redirects the build output log to a
file, keeping just Buildroot's own messages, with the date+time added
at the start of the line.

We need to unbuffer the output of make so that, when the script is
interrupted (SIGINT, ^C), there is no lingering output not yet digested
by the logger loop.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Ricardo Martincoski <ricardo.martincoski@gmail.com>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>