mpy-cross: Add -s option to specify the embedded source filename.

.mpy files contain the name of the source file that they were compiled
from.  This patch adds a way to change this name to an arbitrary string,
specified on the command line with the -s option.  The default is to use
the full name of the input filename.

This new -s option is useful to strip off a leading directory name so
that mpy-tool.py can freeze packages.