Simplify sixlowpan initialisation

[miri64]

By now there are three functions to initialize a 6LoWPAN node

• for a general 6LoWPAN node (6LN) (calls sixlowpan_lowpan_border_init() if as_border == 1)

void sixlowpan_lowpan_init(transceiver_type_t trans, uint8_t r_addr,
                           int as_border);

• for a 6LoWPAN router (6LR) (calls sixlowpan_lowpan_init(trans, addr, 0) in the end)

void sixlowpan_lowpan_adhoc_init(transceiver_type_t trans,
                                 const ipv6_addr_t *prefix,
                                 uint8_t r_addr);

• for a 6LoWPAN border router (6LBR)

uint8_t sixlowpan_lowpan_border_init(transceiver_type_t trans,
                                     const ipv6_addr_t *border_router_addr);

The easiest way to simplify this would be to unify this into one function with flags to tell it what to do.

[miri64]

Won't make it to next release? Reassign?

[mehlis]

part of #644

[OlegHahm]

644 is not yet merged.

[miri64]

But now.