kernel/mbox: add missing dummy thread timeout init

It was possible for a dummy thread to be not timing, but not having
timeout.delta_ticks_from_prev not be -1 at the same time, which is a big
no-no.

Use _init_thread_base() to do a full initialization of the dummy thread.

Fixes ZEP-1312.

Change-Id: I16a2373be3329c142cf26f5dca6bfdbe6014ac5e
Signed-off-by: Benjamin Walsh <benjamin.walsh@windriver.com>