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.

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.

fix unexpected behavior:
reference #9949
reference #10294

fix redefinition for AUTO_INIT_LED0

The board does not have external low speed crystal.

Page 20 of User Manual: 6.10.2 "OSC 32 kHz clock supply"

 * X3 Crystal (not provided)

https://www.st.com/resource/en/user_manual/dm00063382.pdf

This fixes `example/default` and makes `tests/periph_rtc` work properly.

Add common flashing notes + move the doc in a separate doc.txt file

For the moment openocd doesn't work when softdevice blob module is loaded

- Moved code for periph_conf of all ATmega based boards to boards/common/atmega
- Added possibility to override config from individual board:
  - Named file `periph_conf_atmega_common.h` and let this be included from
    `board/$BOARD/include/periph_conf.h` to allow modifications
  - Guarded individual periph configs by `#ifndef $PERIPH_NUMOF` ... `#endif`

The board should not globally set `TERMFLAGS` without a RIOT_TERMINAL
condition.

Also the board does not export uart through usb and thus needs an uart
converter connected. Other boards in the same situation do not need
to set `TERMPROG` without a valid baudrate. So remove the definition
anyway.

This also removes the export of TERM* variables and should make it work
with the other RIOT_TERMINAL.

TERMPROG and TERMFLAGS variables do not need to be exported as they are
used directly by a make receipe.

The link to nodemcu-devkit-v1.0 was missing 'h' in 'http'.

Other links were missing 'http://' or 'https://' and were pointing
to non-existing components on RIOT's web site.

Signed-off-by: Yegor Yefremov <yegorslists@googlemail.com>

Features must be provided by the board if they're actually
available on board. Other features might be provided by the
CPU.

Some grouping is also removed as it is not necessary.

Additionally, it removes unnecessary exports and cleans up
waspmote-pro toolchain variables (not needed) which are taken
from atmega_common.

Leverages common flasher (avrdude) and removes unnecessary exports.
Moreover, a reuse of serial.inc.mk is perfomed from the same
atmega_common/Makefile.include

This commit also change the STDIO UART to use the UARTE device type (with EasyDMA)

- ATmega based boards: Added parameter for W5100 to board_common.h
- Arduino Due based boards: Use board.h instead of providing w5100_params.h

The periph_dma is not pulled in automatically for all applications.
Applications willing to use the configured peripherals with periph_dma
for a given board will have to include the feature explictly in their
Makefile.

It was causing unrelated issues as threads got de-scheduled while
calling printf and this was not handled properly in tests at that
moment.

RTC and RTT are available and already configured for this board but not provided by the build system