net: lwm2m: Fix Server Object SSID access

Server Object SSID should only have Read access.
LightweightM2M-1.1-int-256 confirmance test validate
that write operation to SSID should return error.
Overwrite SSID affect dead block for lwm2m engine and
only reset will heal.

Fix by adding bootstrap overwrite access for Security and
Server object when bootstrap is active.

Signed-off-by: Juha Heiskanen <juha.heiskanen@nordicsemi.no>