`#ifdef FEATURE_PERIPH_NAME` usage should now be replaced by
`#ifdef MODULE_PERIPH_NAME`.

This deprecation was started by pull requests #8226 and #8227.

boards/teensy31 : initial support of Teensy 3.1 & 3.2

nanocoap: fix byte order of request message ID, and add test application

core: optimize bitarithm_lsb()

pkg/micro-ecc: Update FEATURE_PERIPH_HWRNG -> MODULE_PERIPH_HWRNG 

gnrc_ipv6: gnrc_icmpv6: simplify access to network interfaces

tests/gnrc_ipv6_ext: Increase ipv6 addr numof

fixed wrong makefile variable name

boards/common/nucleo: use inverted gpio for user button

boards/remote: invert button logic in SAUL

net/gcoap: Confirmable request with piggybacked ACK response

boards/mulle: fix doxygen invalid use of '@brief'

pkg: disable implicit fallthroughs errors in tinydtls

gcoap: check if NDEBUG is defined

To make sure the cf variable doesn't trigger an `unsed variable` warning
when compile with `-DNDEBUG`.

boards: add support for nrf52-based RuuviTag

Also, create resend_bufs array to store PDU for resend.

stm32l4: implement stmclk interface