scripts: Keep appending log between build parts

Instead of wiping the log file for each make target, only wipe it
initially, and then continue to append to it.  Instead of closing
between files, just flush.  Although the last use doesn't ever close
the file, it is flushed, and will be closed on exit.

Change-Id: Ic4b38cdad993bc32711f832ce30ca8d8dead0db6
Signed-off-by: David Brown <davidb@codeaurora.org>