makefiles/docker.inc.mk: export RIOT_CI_BUILD to docker

The variable was not given to docker before and as it is setting
'RIOT_VERSION_OVERRIDE=buildtest' it helps testing in docker more
consistently.

sys/hashes/sha256: replace include by stddef.h

test/xtimer_drift: interval calculation

cpu/cc2538: enhance periph ADC

stddef.h is enough for size_t, unistd.h adds a lot of other declarations

sys/hashes/sha256: add missing unistd.h include

It needs `size_t` definition.

tests/gnrc_netif: add test for ULA source selection

make: unify app folder search (examples/*, tests/*, ...)

atmega_common: allow defining rom and ram length for link.

Size is from Francisco knowledge on stk500v1.

Value found by checking fuse settings

Value comes the maintainer.

Taken from https://store.arduino.cc/arduino-uno-rev3

Taken from https://store.arduino.cc/arduino-mega-2560-rev3

Taken from https://www.arduino.cc/en/Main/ArduinoBoardDuemilanove

It is defined using the `ROM_RESERVED` variable.

This should allow supporting arduino bootloader that is stored at the
end of the ROM.

Now all atmega cpu define them so use them by default.

Info taken from https://www.microchip.com/wwwproducts/en/atmega1284p

TODO: update "board not enough memory".

Info taken from https://www.microchip.com/wwwproducts/en/atmega256rfr2

TODO: update "board not enough memory".

Info taken from https://www.microchip.com/wwwproducts/en/atmega328p

TODO: update "board not enough memory".

Info taken from https://www.microchip.com/wwwproducts/en/atmega1281

TODO: update "board not enough memory".

Info taken from https://www.microchip.com/wwwproducts/en/atmega2560

TODO: update "board not enough memory".

This allows configuring __TEXT_REGION_LENGTH__ and __DATA_REGION_LENGTH__
for previous versions (ubuntu xenial for example).

Generated by extracting the output of

    avr-gcc -Wl,--verbose --mmcu=$(CPU)

With avr-binutils 2.26

Allow configuring __TEXT_REGION_LENGTH__ and __DATA_REGION_LENGTH__ linkerscript
variables using ROM_LEN and RAM_LEN makefile variables.

README: update listing support of ESP8266

This helper Makefile is supposed to unify finding folders with
applications that can be built.