pkg/spiffs: update pkg to v0.3.7

cleanup: fixing (some) special chars

sys: xtimer: add xtimer_set_timeout_flag()

spelling: s/initialisation/initialization/

spelling: s/initialise/initialize/

boards/iotlab-common: fix clock configuration

sys/net: fix typo on word 'initialized'

cpu/sam0: optimizations to the shared UART driver

drivers: doxygen cleanup

doc: added LOSTANDFOUND.md

drivers: add driver for apa102 RGB LEDs

doc: Enable REPEAT_BRIEF in Doxygen

Atmega fix typo and empty space

examples/default: the saml21-xpro has no radio...

Added means to document (temporarily) removed features. The file
contains a list of all modules/features that were removed for
various reasons (typically: not maintained anymore), so that these
can be reintegrated in a later point of time.

cpu: doxygen cleanup

mrf24j40: rewrite binary constants to hex

cpu/stm32_common: fix PLLs enabling and usage

core: thread_flags: Set sched_status_switch_request after waking a waiting thread

cpu/stm32f[0|1|3] unify stmclk with stm32_common