This currently does nothing but setting FLASHFILE when flashing.
This will allow passing the variable when flasher will use the FLASHFILE
variable.

FLASHFILE is now a generated file when doing `make all`.
This prepares also for when flashers will use `FLASHFILE` as a file to
be flashed.

It currently still needs the hack below for openocd and edbg.

This also fixes the issue when building 'riotboot' in docker that was
being built with the host toolchain.

Using 'link' was working too but will introduce a circular dependency
when FLASHFILE is one of the slot files.

This trims down to the minimal required dependency to work. It is now
the same as `ELFFILE` dependencies.

Include the 'riotboot.mk' file before using FLASHFILE/ELFFILE/HEXFILE
variables. This will allow setting variables to values from riotboot.mk
like `FLASHFILE = $(RIOTBOOT_COMBINED_BIN)` before it is evaluated in
Makefile.include.

It should be included after defining 'BINFILE' for 'riotboot.bin'
handling.

Use the new variable instead of the composed name.

serial.inc.mk: disable local echo in picocom.

cpu/nrf52/nrf802154: unify address generation

makefiles/murdock.inc.mk: do not overwrite FLASHFILE if set

cpu/nrf52/radio: initial support for nrf52's ieee802.15.4 radio

If FLASHFILE is set keep the original value.

It changes the variable from an immediate to a deferred variable but if
murdocks keeps working there is no issue.

boards/common/stm32: rework common clock configuration for stm32f4

codacy: disable complaining about python assert

Makefile.include: introduce FLASHFILE

This silents the reported issue in codacy about python asserts:

    Use of assert detected. The enclosed code will be removed when
    compiling to optimised byte code.

The concern is valid about python asserts, but they are used in tests
and python is not run with optimised byte code.

Solution taken from codacy website
https://support.codacy.com/hc/en-us/articles/207994335-Code-Patterns

makefiles/tools: Improved avrdude integration

The CPU variable in the boards Makefile.include file already contains the target
CPU, so there is no reason to provide it in each board again as avrdude flag.

This commit automatically sets the avrdude target from the CPU variable and
removes the unneeded flags.

Currently the flag "-P ${PORT}" is added to avrdude regardless of the programmer
used. But this flag should only be set for programmers that operate over a
serial port - e.g. like the various Arduino bootloaders. This commit changes
the behaviour so that the "-P flag" is only set for only of the default
programmers of the various AVR boards supported by RIOT. This allows to use
ICSP programmers (e.g. like the usbtiny) like this:

    make BOARD=arduino-uno PROGRAMMER=usbtiny

Introduce FLASHFILE variable to start migrating boards to use it.

This is the file that will be used for flashing.
Boards do not currently use it but will migrated in upcoming PRs.

This fixes the boards being stuck when initializing the RTC

A common configuration file is introduced for stm32f4 with core clock
at 168MHz with HSE at 8MHz, 2 configuration files are introduced to²:
distinguish between clock configuration with and without LSE.

tests/leds: initialize led0 by default

fix unexpected behavior:
reference #9949
reference #10294

fix redefinition for AUTO_INIT_LED0

pkg/wakaama: bump version

- copy basic objects from client implementation

- fix pkg warnings

- use lwm2m_strdup instead of strdup

- fix alignment problem in lwm2m data struct

- add fix of acc_ctrl object read

boards/stm32f3discovery: Fix no external low speed crystal

boards/lobaro-lorabox: Remove auto init LED0

net/nanocoap: don't abort server on recv error

 tests/bloom_bytes: replace %f with fmt/print_float

dist/pythonlibs: provide unittest TestCase wrapper for testrunner

 gnrc_netif: check all required options on initialization

With `DEVELHELP` activated all required options required by GNRC are
now checked at interface initialization, so that developers of new
link-layer protocols or device drivers notice as soon as possible that
something is missing.