zephyr: run server in blocking mode in the main thread

• There are problems when running the server in a separate thread on arduino nano 33 ble, while it works on qemu_x86:

<err> os:   Stacking error (context area might be not valid)
<err> os:   Data Access Violation

Therefore for now we are running the server thread in the main thread in blocking mode.

• CoapSite::set_sock is now static and move set_sock call from start_server into CoapServer constructor
• Add check if no resource directory is found in the network then stop execution of the program.

Signed-off-by: Francesco Pham francesco.pham@huawei.com