Move message handler setup to separate task

The OpenAMP library has changed how the wait_remote_ready functions work
for Remoteproc and RPMsg, instead of using metal_yield while waiting for
the remote side to get ready, they now use metal_sleep_usec.

This causes issues in the message handler implementation because
FreeRTOS implements the sleep function using vTaskDelay, which can only
be used after the scheduler has been started and the message handler
performs its Remoteproc setup before starting the scheduler.

To solve this, the Remoteproc setup has been moved to its own task that
will be run before any other task. This allows the use of vTaskDelay
during the setup.

Change-Id: Idf9e86e62ff0ff4bd716d33987cb70bc34743229
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>