bootstrap, first step implementation

You can add a bootstrap server, and trigger a bootstrap.
A 'bootstrap' command is added to the client.

Bootstrap doesn't use a transaction because currently transaction.c
works only with lwm2m_server now.
Bootstrap won't work now because the bootstrap server will try to DELETE
the whole /1/ hierarchy and provision new servers which is cleary not
supported.

Next step for me is to implement bootstrap in Leshan for having a clear understanding of the whole process. Then I'll be back tackling the remaining issues.