python-m2crypto: requires host-python2

Acording to its homepage [1], m2crypto does not support python3 yet.

So, this patch forces building the host m2crypto module for pyhton2.

[1] https://pypi.python.org/pypi/M2Crypto



[Thomas: take into account the rename of the new variable to
<pkg>_NEEDS_HOST_PYTHON.]

Signed-off-by: Samuel Martin <s.martin49@gmail.com>
Cc: Gustavo Zacarias <gustavo@zacarias.com.ar>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>