Fixes #2806.

stmhal will now be built by default with frozen bytecode from scripts
stored in the stmhal/modules/ directory.  This can be disabled or
changed to another directory by overridding the make variable
FROZEN_MPY_DIR.

This is how CPython does it, and it's very useful to help users discover
the available modules for a given port, especially built-in and frozen
modules.  The function does not list modules that are in the filesystem
because this would require a fair bit of work to do correctly, and is very
port specific (depending on the filesystem).

Ports should no longer use pyhelp_print_obj but instead should define
MICROPY_PY_BUILTINS_HELP to 1 and then specify their help text using
MICROPY_PY_BUILTINS_HELP_TEXT.

This builtin is configured using MICROPY_PY_BUILTINS_HELP, and is disabled
by default.

For a small number of bytes, it's expected to return a small int without
allocation.

This test works only for MICROPY_LONGINT_IMPL == MICROPY_LONGINT_IMPL_MPZ
and needs a way of skipping in other cases.

To be implemented later.

If result guaranteedly fits in a small int, it is handled in objint.c.
Otherwise, it is delegated to mp_obj_int_from_bytes_impl(), which should
be implemented by individual objint_*.c, similar to
mp_obj_int_to_bytes_impl().

This makes MicroPython app running in QEMU be pingable from the host (by
following QEMU networking setup instructions,
https://www.zephyrproject.org/doc/samples/net/qemu_setup.html).

In anticipation of enabling more features in the default build. Also, fix
compilation of minimal build.

If, for example, the code-size check failed we would still like to run the
coveralls coverage analysis.

The if-block that this unreachable code is in has a condition "f>=5" so
"fp_isless1(f)" will always fail.

This implements flow control in case user does not call recv method often
enough (it tells host side to stop sending more data).