package/python3-pyelftools: new package

We will need this Python 3.x variant of the host-python-pyelftools
package to be able to build some recent versions of U-Boot (>=
2020.01).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>