Fix #549

[arnopo]

This Pull request is a fix of the PR #549
The rvdev->notify_wait_cb is never initialized to NULL if not set by the user.
the proposal to fix is to initialize it to NULL and provide an API for the user to set it.

The issue is reproduced using flood-ping test in linux environment.

echo "################### run flood test #####################"
LD_LIBRARY_PATH=./bin/usr/local/lib:../../libmetal/build/lib/  ./bin/usr/local/bin/rpmsg-echo-shared &
sleep 1
LD_LIBRARY_PATH=./bin/usr/local/lib:../../libmetal/build/lib/  ./bin/usr/local/bin/msg-test-rpmsg-flood-ping-shared 1 || exit

@wyr8899 , pleas could you review it and give a feedback

[edmooring]

Looks good to go.

[arnopo]

@xiaoxiang781216 : could I consider that if it is ok for you it is also ok for @wyr8899 ? The initial PR was related to NuttX, right?

[xiaoxiang781216]

yes, @wyr8899 and @CV-Bowen please verify the change.

[wyr-7]

@arnopo @xiaoxiang781216 Sorry, I changed my account for some reasons and it is now approved.