But now it's generic ring buffer implemented via ringbuf.h, and is intended
for any type of input, including dupterm's, not just UART. The general
process work like this: an interrupt-driven input source puts data into
input_buf, and then signals new data available via call to
mp_hal_signal_input().
Enabling standard assert() (by removing -DNDEBUG) produces non-bootable
binary (because all messages go to .rodata which silently overflows).
So, for once-off debugging, have a custom _assert().
Initialize RTC period coefficients, etc. if RTC RAM doesn't contain valid
values. time.time() then will return number of seconds since power-on, unless
set to different timebase.
This reuses MEM_MAGIC for the purpose beyond its initial purpose (but the whole
modpybrtc.c need to be eventually reworked completely anyway).