thread_analyzer: Allow to override the auto-analysis thread priority

By default the thread priority is still the lowest application thread
priority. Allow the priority to be increase by specifying both, the
`override` option as well as the desired `priority`. This pattern was taken
from the shell thread.

Signed-off-by: Cla Mattia Galliard <cla-mattia.galliard@zuehlke.com>