tools/makemanifest.py: Fix build on Windows by adding .exe to mpy-cross.
tools/makemanifest.py: Fix build on Windows by adding .exe to mpy-cross.

When using a manifest on Windows the reference to mpy-cross compiler was
missing the .exe file extension, so add it when appropriate.

Also allow the default path to mpy-cross to be overridden by the (optional)
MICROPY_MPYCROSS environment variable, to allow full flexibility on any OS.

pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython,pfalcon/micropython