2. Add flag INTERNAL_LIBM to mpconfigboard.mk files to signal using lib/libm.
3. Fix floating-point warnings in lib/libm
4. Turn off -finline-limit, no longer needed due to 1.
5. Add explicit flags for SPI_FLASH_FILESYSTEM and INTERNAL_FLASH_FILESYSTEM
to mpconfigboard.mk files, so they don't need to know the name of the .c file
that implements those. Makefile takes care of checking the flags and including
the right files.
This saves a lot of RAM. Fixes#287.
Also fixed compilation of frozen_mpy.c to use supplied make rule rather than
builtin rule (supplied rule suppresses printing out the gcc command line).
Reworked frozen module support: clean up makefiles and handle multiple directories.
Modules to freeze are included as git submodules.
Add neopixel to circuitplayground express build.
Fixes#56
