Make: do not spam error messages on shallow clones

> `--always`: Show uniquely abbreviated commit object as fallback.
Add the ability to send a message to the current thread's message queue
(without raising an error)
ccnl: LRU is the replacement strategy, rename variable to express this
native: optionally exit() after last thread has ended

Used when compiled with NATIVE_AUTO_EXIT defined.
Merge pull request #972 from kaspar030/native_exit_after_last_task_v2

Make native exit after last thread has ended
Merge pull request #1003 from mehlis/ccnl-lru

ccnl: LRU is the replacement strategy, rename variable to express this
Merge pull request #990 from Kijewski/allow-shallow-clone

Make: do not spam error messages on shallow clones
Merge pull request #970 from rousselk/send_msg_intra_thread

core: Add the ability to send a message to the current thread's message queue
Proposal for common 802.15.4 radio driver API definition.
Added separate load_tx() and transmit() functions behind send()
plus corrected some errors and completed documentation
Corrected documentation
Completed prototypes for send() and load_tx() functions,
and added the corresponding ieee802154_node_addr_t type