build_msg: Automatically fill out the build name and log

We use regularly formatted names for the build logs on the build host
and currently (other than the final build) only run one build at once
so rather than requiring the user to fill in the blanks on which build
has failed and copy the log over by hand we can have the script fill
these bits in for us.

This only works properly for kernel builds, for KUnit and kselftest the
logs don't have a quiet mode so manually copying makes more sense.  I'll
deal with handling that when I have an example to hand to test.  It'll
also fall over if we have multiple builds at once.

Signed-off-by: Mark Brown <broonie@kernel.org>