package/qpid-proton: needs host-python2

qpid-proton is using constructs that are not valid in python3, so we
must use a host python2 interpreter. To make this happen this commit
adds a dependency on host-python and explicitly tells CMake to use
$(HOST_DIR)/usr/bin/python2.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Baruch Siach <baruch@tkos.co.il>
Cc: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>