read_attr_value() function

* Function read_attr_value * Description Utility function to read an attribute value. * Parameter p_attr: pointer to the attribute to read. * offset: read offset. * p_value: output parameter to carry out the attribute value. * p_len: output parameter to carry out the attribute length. * read_long: this is a read blob request. * mtu: MTU * sec_flag: current link security status. * key_size: encryption key size. * Returns status of operation.