Makefile.uk: Add rule for creating root filesystem

Makefile.uk: Add rule for creating root filesystem

The Python interpreter needs a filesystem where to keep its libraries. This
patch creates a Python virtual environment and installs the Python 3 standard
library in /lib/python3.7 directory. For creating the filesystem the following
command has to be run:

$ make python-rootfs path=<directory>

Signed-off-by: Costin Lupu <costin.lupu@cs.pub.ro>
Reviewed-by: Felipe Huici <felipe.huici@neclab.eu>