This commit simplifies the customisation of the main MicroPython execution
loop (4 macros are reduced to 2), and allows a board to have full control
over the execution (or not) of boot.py and main.py.
For boards that use the default start-up code, there is no functional
change in this commit.
Signed-off-by: Damien George <damien@micropython.org>
7b41d7f187