utils/brmake: filter output for parallel build

When building in parallel with per-package directories
(BR2_PER_PACKAGE_DIRECTORIES=y), brmake output is often garbled:

  2025-10-08T18:39:10 >>> host-dtc 1.7.2 Building
  2025-10-08T18:39:11 checking for stdint.h... >>> host-dtc 1.7.2 Installing to host directory
  2025-10-08T18:39:12 checking for limits.h... >>> host-gmp 6.3.0 Installing to host directory

Remove the spurious string between the timestamp and the ">>>" marker to
fix this.
We need some extra care to preserve the preceding "term bold" special
characters sequence.

We also prevent grep and sed to buffer their output too much. This leads to
more frequent output even when we might not be connected to a terminal; for
example: when brmake's output is piped to another program or when running
in CI.

Reviewed-by: Marcus Hoffmann <buildroot@bubu1.eu>
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Julien Olivain <ju.o@free.fr>
(cherry picked from commit c9dca7f4c3)
Signed-off-by: Thomas Perale <thomas.perale@mind.be>

utils/brmake

@@ -12,7 +12,8 @@ main() {
              printf "%(%Y-%m-%dT%H:%M:%S)T %s\n" -1 "${line}"
          done \
          |tee -a br.log \
-         |grep --colour=never -E '>>>'
+         |grep --line-buffered --colour=never -E '>>>' \
+         |sed --unbuffered -E 's/^([0-9T:-]{19}) [[:print:]]*(.*>>> )/\1 \2/'
        )
     ret=${?}