Useful for embedded targets with fixed stack layout.

NameError may either include offending name or not. Unfortunately, this
change makes test float-dependent. And using integer division leads to
different error message than CPython.

GPIO16 is actually special-function I/O, though some boards have LED there.

OPEN_DRAIN is of course synthesised.  All pin modes are tested and
working.

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).

Framework for time.ticks_cpu added, but not implemented.

This is to get the test suite running and passing.

Otherwise, os.uname() returns empty string for SDK version.

For some reason, Travis now has Google Chrome PPA included in the builder
image, that lacks i386 arch, that leads to apt-get update error. So, ignore
it (this is not ideal as may lead to actual repo update failures to be missed,
leading to installation of old package, leading to weird errors; let's keep
that in mind).