boards/samr21-xpro:fix SPI_1 DIPO config

cpu/saml21: Conditionally define LITTLE_ENDIAN

drivers/dht: fix fake parsing error with DHT22

cpu/stm32: fix header include guards

boards/waspmote-pro: porting for new libelium's Waspmote PRO v1.2 platform

Caused problems with redefined macros when building with a recent newlib

samd21: do not redefine LITTLE_ENDIAN

cpu/stellaris: fix header include guards.

doc: eliminate clutter in adc and gpio periph docs

RE-Mote revision A update

cpu/x86: fix header include guard.

cpu/samd21: remove include instance_sercom3.h

tests/thread_exti: add small README

examples/microcoap_server: fix .well-known/core endpoint

tests/xtimer_longterm: fix expected delays

Clang: Add support for C++

examples: add xbee support for gnrc_minimal

cortexm_common: Fix isr_stack usage count

That include will be already picked up by cpu.h -> cpu_conf.h -> samd21.h

cpu/atmega*: unify common code for atmega CPUs

CPU：Fix header include guards

Refactor and add multiply by word size to get the usage in number of
bytes instead of in number of words.

Verified implementation by manual memory inspection in GDB.

cortexm_common: move the stack to the bottom of RAM