tools: new tool to filter the output of make

tools: new tool to filter the output of make

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" <[email protected]>
Cc: Maxime Hadjinlian <[email protected]>
Cc: Arnout Vandecappelle <[email protected]>
Cc: Ricardo Martincoski <[email protected]>
Cc: Samuel Martin <[email protected]>
Signed-off-by: Thomas Petazzoni <[email protected]>