sys/auto_init/saul: fix of #10098

CONTRIBUTING: rewrite the existing document

boards/sensebox: Add limitations of pin multiplexing

This describes the limitations of the interrupt pin being multiplexed.

Fixup: add details of specific pins

doc/doxygen: fix typo in getting started page

cflags: use gnu99 only if nothing else is specified (mips, msp430, native)

cpu/efm32/gpio: use gpio_irq feature

dist/tools/jlink.sh: fix testrunner on stdio_rtt based devices

gnrc_netif: Use fmt for bytes to hex string

cpu/stm32f1/gpio: use gpio_irq feature

gnrc_netif_ieee802154: rework `netif->state` to getter and flag

drivers/bmx280: improve documentation

tests/periph_flashpage: add automated test for flash peripheral

This replaces the netif->state accesses in netif::recv with a netdev call
and the new netif RAWMODE flag

Adds a gnrc_netif specific rawmode flag to indicate that the netdev
device is configured in raw mode. This flag is kept in sync with a
possible flag in the netdev device and should only be modified via the
setter call.

netdev: fix return value and precondition doc

While reviewing #9942 I noticed that the documentation on the netdev
driver API is unclear and in some cases outright contradicting itself:

> ```
> @return              number of bytes used from @p value
> @return              `< 0` on error, 0 on success
> ```

IMHO this is unacceptable for such a central API where communication

This fixes a few things and also clarifies preconditions:

- Specifies negative `errno`s clearly so all drivers return the same
  when required
- Re-iterates parameter preconditions and special cases within the
  parameter documentation itself (might also help towards #9805?)
- Fixes contradictions within return value documentation
- Adds missing parameter documentation on `init()`.

- Add minimal usage instructions
- Ensure that the `bme_280_read_humidity` function (only available on
  the BME280) is included in the documentation.
- Fix a typo

MAINTAINING.md: Improve review process between multiple maintainers

tinycbor: add doc.txt file

During RIOT Summit 2018 the attending maintainers found that the
organization for the review process involving multiple maintainers is
lacking both in process description and existing mechanisms.

This changes the "Organisation of reviewing between maintainers"
section of the maintenance guidelines, to be in accordance with the
results of the discussions spawned from this fact.

If approved the following labels are added to GitHub:

- Reviewed: 1-fundamentals
- Reviewed: 2-code-design
- Reviewed: 3-testing
- Reviewed: 4-coding-conventions
- Reviewed: 5-documentation

boards: add samr30-xpro board and samr30 cpu

Co-authored-by: biboc <bapclenet@gmail.com>