machine.POWER_ON is renamed to machine.PWRON_RESET to match other
reset-cause constants that all end in _RESET. The cc3200 port keeps a
legacy definition of POWER_ON for backwards compatibility.
To let unix port implement "machine" functionality on Python level, and
keep consistent naming in other ports (baremetal ports will use magic
module "symlinking" to still load it on "import machine").
Fixes#1701.
py/mphal.h contains declarations for generic mp_hal_XXX functions, such
as stdio and delay/ticks, which ports should provide definitions for. A
port will also provide mphalport.h with further HAL declarations.
