python-cheetah: add host-package support

Some packages, like GNURadio for VOLK, needs cheetah on host at buildtime.

The dependency on host-python-markdown is needed to avoid that setuptools
downloads markdown if it is not installed yet.

[Thomas: add comment in the .mk file.]

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Samuel Martin <s.martin49@gmail.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>