Linker limitation: the native module is not linked against the symbol table of the

full MicroPython firmware. Rather, it is linked against an explicit table of exported

symbols found in ``mp_fun_table`` (in ``py/nativeglue.h``), that is fixed at firmware

build time. It is thus not possible to simply call some arbitrary HAL/OS/RTOS/system

function, for example.

New symbols can be added to the end of the table and the firmware rebuilt.

The symbols also need to be added to ``tools/mpy_ld.py``'s ``fun_table`` dict in the

same location. This allows ``mpy_ld.py`` to be able to pick the new symbols up and

provide relocations for them when the mpy is imported. Finally, if the symbol is a

function, a macro or stub should be added to ``py/dynruntime.h`` to make it easy to

call the function.