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>

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

boards/hifive1: include cpu/fe310 features

board/mips-malta: include the cpu features instead of common cpu

tinycbor: Version bump to 0.5.2

This release fixed some bugs related to disabling float support in the
parser.
The float support is now optional and can be enabled by including the
tinycbor_float module. This allows for compiling on AVR as that platform
doesn't support the float specific defines used by tinycbor.

gnrc: Use existing utility function to extract IPv6 header

gnrc_sock_recv used to duplicate functionality of gnrc_ipv6_get_header,
but additionally checked whether the IPv6 snip is large enough.

All checks are now included in gnrc_ipv6_get_header, but as most of them
stem from programming / user errors, they were moved into asserts; this
constitutes an API change.

color: Fix -Wdouble-promotion warnings

native/netdev_tap: Rename variable to fix -Wshadow warning

sys/ecc: move hamming256 module to generic ecc

net/nanocoap: factor out generic CoAP defines

cpu/esp8266: remove duplicate 'CPU' variable

cpu/esp8266: periph/i2c fix

The board should process its CPU features.
This has the consequence to make 'periph_pm' visible to the board.

I also removed the duplicate FEATURES_PROVIDED from the board defined in
the cpu.

This change was missed when the cpu 'mips32r2_generic' was separated
from 'mips32r2_common'. The target file already includes 'mips32r2_common'.

doc: add `memory management` group

boards/openmote-b: Fix RF_SWITCH_* logic to active low

RF_SWITCH_2_4_GHZ_ON should turn PD4 low not high to active the 2.4 GHz radio.
The same goes for RF_SWITCH_SUB_GHZ_ON on PD3.

cpu/nrf5x/gpio: use periph_gpio_irq feature

pkg/semtech-loramac: RU864 band added

cpu/cc2538/gpio: use gpio_irq feature