gnrc_netreg: Revert #8517 partly

make: add flag for no-implicit-fallthrough in CXXFLAGS

cpu: efm32: fix typo in cpus.txt

sys/can: fix compiler warning due to usage of potentially uninitialized variables

This reverts commit c2b403f4.

This reverts commit a7b77893.

This reverts commit c178ea87.

nanocoap: add context pointer to resources

boards/b-l072z-lrwan1/dist: fix openocd reset_config options

boards: stk3600: add support (v2)

drivers/sx127x: Fix -Wmaybe-uninitialized warning

drivers/saul: add UV and distance sense options

pkg/openthread: cleanup makefile + fix implicit-fallthrough warning

tests/driver_sx127x: fix setup issue

makefiles: always link with GCC even if TOOLCHAIN is set to LLVM

drivers: fix indentation in Makefile.include

This variable can be used to specify the linker which should be used to
link C++ object files. This is needed in order to use g++ for linking
with TOOLCHAIN=llvm.

Fixes #8356

pkg: fix compile errors in gecko_sdk and umorse

- soft_spi was using a 4 spaces indentation, but only 2 are required

tests/emb6: remove obsolete FEATURES_REQUIRED from makefile

make: always set -Wno-implicit-fallthrough