document how to work with SWD with WFI_IDLE enabled
merging RTC helpers from my experimental branch
merging RTC helpers from my experimental branch, these go pretty much hand-in-hand with the lowpower stuff
Merge branch 'chibios_lowpower' of github.com:rambo/ruuvitracker_fw into chibios_lowpower
Merge pull request #32 from rambo/chibios_lowpower

low-power modes, entering and recovery, RTC shell commands for testing wakeup.
Do not block on _DEBUG() when USB is not connected.
merge upstream