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.
Such functions are never used after MicroPython has started, and they
remain in RAM wasting space. Now they are placed in a special section
named "boot" which sits just before the heap, allowing us to extend
the effective heap area up to the new boot section. Right now, this
gives us back ~1K, but in the future, more functions might end up in
there as well.
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.
