Fixes to lwIP implementation from external projects.

"Mailboxes can be created in a different process than the one that waits
for messages. This is a problem with BeRTOS MsgPorts because the process
to be woken up is set during init.
In this patch, the process to be awaken is changed just before
waiting.

Fix compilation warnings."

git-svn-id: https://src.develer.com/svnoss/bertos/branches/2.7@4994 38d2e660-2303-0410-9eaa-f027e97ec537