saul/gpio: Low footprint handling of active-low signals + state initialization

cpu, sam0_common: fix uart TXC check

drivers/sx127x: several fixes

cbor: fix integer overflow in decode_bytes

cpu/stm32_common: use pseudomodule for UART hardware flow control

xtimer: Set XTIMER_SHIFT to 0 if underlying timer is 32768 Hz

nrfmin: port for gnrc_netif2

gnrc_ipv6_nib: return registration state on upstream registration

The function `_update_nce_ar_state()` was introduced during the review
of \#7424, but it's return value never used, causing faulty behavior.

gnrc_netif2: Introduction of a new GNRC network interface API

nib: implement public NIB functions up to link-local AR

kinetis: Adjust RTT alarm trigger time

cpu/sam0/periph/uart: ensure uart_init() idempotency

cpu/stm32f0: optimized isr vector definitions

cpu/stm32f7: optimized isr vector definitions

cpu/stm32f4: optimized isr vector definitions

CPU: add sam_common with CMSIS headers

cpu, sam0_common: update vendor headers

cpu/stm32f2: optimized isr vector definitions

gnrc_ipv6_nib: add forwarding table component