tests: let license checker check headers, too
make: don't call mkdir -p a lot
make: use $@ and $<
make: call ranlib
make: don't invoke wildcard repeatedly
make: easier to read object targets
core: introduce KERNEL_PID_FIRST and KERNEL_PID_LAST
core: let valid PIDs start with 1
unittests: do not use auto_init
core: clist is a cyclic list
core: clist without explicit thread

Right now the core component `clist` is a generic cyclic doubly-linked list.
In the core it is used in `tcb_t::rq_entry`.
Further it is used `net_if.c`.

This commit removes the member `clist_node_t::data` which stored the
pointer to the `tcb_t` instance of which the clist is already a member.
The needless member added `sizeof (int)` bytes to every instance of
`tcb_t`.

In `net_if.c` the clist was used in a type-punned way, so that the
change won't affect it.
examples: renamed rpl_udp header
examples: cleanup for rpl_udp

* removed unnecessary shell commands
* updated documentation
* Unified global default IP address
rpl: moved headers to sys/net/include
examples: don't advertise IPv6 prefix

Cleaning up the IPv6 address initialization and removed the addition of
an IPv6 prefix to advertise.
examples: use correct broadcast address when sending
fix low-level sending destination address
Merge pull request #1571 from Kijewski/make-mkdir-once

make: cleanup Makefile.base
periph_driver: introduce a RTT interface
periph:rtc: add missing argument for callback