The 512k build does not have a filesystem so there is no reason to include
the filesystem-related modules.  This commit provides a custom manifest.py
for this board which no longer includes: _boot.py, flashbdev.py,
inisetup.py, upip.py, upip_utarfile.py.  This cuts the build down by about
9k of flash.
Signed-off-by: NDamien George <damien@micropython.org>