The previous fix using -T4 is insufficient because queue-based job
distribution does not guarantee one board per thread. Even with 4
threads and 4 boards, a faster thread can finish its board and grab
another from the queue, causing .config to leak between boards in
the same thread's work directory.
Fix by using -P (per-board-out-dir) which gives each board its own
output directory, completely preventing .config leakage regardless
of thread scheduling.
Fixes: 17618b5975 ("buildman: Fix flaky test_kconfig_change test")
Co-developed-by: Claude Opus 4.5 <noreply@anthropic.com>
Signed-off-by: Simon Glass <simon.glass@canonical.com>
9db339213d