This allows to use the On-Chip retention registers for both the
RTC and to share notification flags between the bootloader and the
application. The two flags being shared right now are the "safe boot"
request and the WDT reset cause. we still have 2 more bits free for
future use.
The port currently implements support for GPIO, RTC, ExtInt and the WiFi
subsystem. A small file system is available in the serial flash. A
bootloader which makes OTA updates possible, is also part of this initial
implementation.
