[communication] Fixes several bugs and modifies return values.
* Corrects "mutex lock" in sb acceses to the central buffer (one could slap me for this).
* Modifies return value on I2C_receive.
* Adds node_cfg.h include in debug lib, to enable local settings for debug default address.
* Adds a retry mechanism for sb_forward.
* Adds the corresponding define in several node_cfg.h.