Add a documentation page for advanced low level tricks.
Document the `RIOT_MAKEFILES_GLOBAL_PRE` and
`RIOT_MAKEFILES_GLOBAL_POST`.

I could not work around to have `$(RIOTBASE)/Makefile.include` in the
doc as the `$()` part was removed, so I kept with `$RIOTBASE` for now.

This allows specifying a list of files that should be parsed by make at
the beginning and at the end of Makefile.include.

It is a generic mechanism to allow specifying system wide configuration:

* Globally overwrite the 'TERMPROG'
* Specify a hard written port / debug_adapter_id for some BOARD values
* Define you own specific targets
* Override default targets

It can include file before and after Makefile.include to allow handling
different configurations.

boards/{firefly,samr30-xpro}: add missing `periph_gpio_irq` provided feature

cpu/esp8266: fix of adc_res_t

make: fix BUILD_IN_DOCKER when RIOT is submodule

periph/uart: fix line length in doc header

tests/xtimer_now64_continuity|xtimer_usleep: disable running test on native

openocd: get flash and debug files from cli

gnrc_netif: make link-layer specific operations proper submodules

mcuboot.mk: Add mcuboot targets to BUILDDEPS.

The test randomly fails for unrelated PRs. Expecting real-time timing on
native is not really possible. Maybe it could be fixed but it currently
triggers many false positive.

The test randomly fails for unrelated PRs. Expecting real-time timing on
native is not really possible. Maybe it could be fixed but it currently
triggers many false positive.

When doing `make -j clean all' the directories can be cleaned after
files are made. To ensure files are created after clean, those targets
are made conditionally dependent on the clean target.

This copies the handling done in Makefile.include.

eclipse.inc.mk: Split eclipse support into it's own file.

Get FLASH_FILE from command line instead of environment variable.

The documentation was claiming ELFFILE was given as a command line argument
already, but is was not.

This is the same change as with `openocd.sh`.

Get FLASH_FILE and ELFFILE from command line instead of environment variable.

The documentation was claiming ELFFILE was given as a command line argument
already, but is was not.

To keep Makefile.include clean and to be consistent with other tools,
the Eclipse IDE support is put in a separate file.

No that we introduced sub-modules to `gnrc_netif` we can move that here
as well.

gnrc_netif_hdr: provide convenience function to get netif pointer from netif header.

drivers/periph_eeprom: add clear and erase functions

shell_commands/ifconfig: mark TENTATIVE addresses correctly

dist/testbed-support: add firefly in IoT-LAB archis