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: Damien George <damien@micropython.org>
