This is to keep the top-level directory clean, to make it clear what is
core and what is a port, and to allow the repository to grow with new ports
in a sustainable way.
The idea behind decrease is: bytecode and other static data is also kept on
heap, and can easily become half of heap, then setting threshold to half of
heap will have null effect - GC will happen on complete heap exhaustion like
before. But exactly in such config maintaining heap defragmented is very
important, so lower threshold to accommodate that.
To start with, the critical scripts _boot.py and flashbdev.py are frozen
to improve performance and reduce RAM consumption.
Saves about 1000 bytes of heap RAM for a bare boot with filesystem.
