dist/tools/doccheck: add check for undefined groups

gnrc_ipv6_nib: use router instead of netif for router timeout context

net/emcute: use global byteorder functions

core/byteorder: add uint16 from/to buffer funcs

drivers/servo: add periph_pwm as dependency

gnrc_netif: increase number of maximum IPv6 groups

gnrc_netreg: Further specify gnrc_netreg_register() precondition

While browsing through the rendered doc, I found the precondition of
`gnrc_netreg_register()` somewhat lacking. What is a "message queue"?
`gnrc_netreg_entry_t`s have types. Does this apply for all types?
This specifies the requirement more: The calling thread **only** needs
a message queue (also provides a link to `msg_init_queue()` now for
further information), if the provided `gnrc_netreg_entry_t` is of type
`GNRC_NETREG_TYPE_DEFAULT` (i.e. thread-wise IPC).

tests/pkg_semtech_loramac: improve README

pkg: add support for the NimBLE BLE stack

`GNRC_IPV6_NIB_RTR_TIMEOUT` [expects a default router entry][doc] as
context, so using the `netif` here is clearly wrong. This causes the
hardfault described in #9297.

[doc]: http://doc.riot-os.org/group__net__gnrc__ipv6__nib.html#ga1c7e892ef6533f699c4e64737c2847c2

rpl: add solicited information option to DIS

tests/xtimer_usleep: improved test, added pin toggle

cpu: efm32: provide non-standard UART modes

boards: slstk3402a: add support

boards: add stm32mindev (clone of the Blue Pill board)

Added stm32mindev to BOARD_INSUFFICIENT_MEMORY where needed

Added support for the STM32 Minimum System Development Board, which is
identical to the Blue Pill except for the GPIO pin wired to the on-board LED.

- Moved common code for STM32F103C8 based boards to the new module
- Updated board `bluepill` to use the new module

core/assert: provide static_assert for c99