generate_config: New `-i` arg for j2 includes dir

generate_config: New `-i` arg for j2 includes dir

jinja2 does not support imports from parent directories, so in order
for j2 to be able to use '{% from <some>.j2 import ... %}' constructs
with included j2 residing on a separate dirpath than current j2, add
support for explicitly adding directories to the templater search
path.

This also allows calling generate_config with only file basename as
'-j' arg value, as long as the dirname is passed via '-i' (can be
extended later to parsing multiple j2 files at once).

Change-Id: I943fda9262a6f7de5ee6fcc3e7bc9828c339651a
Signed-off-by: Alexandru Avadanii <Alexandru.Avadanii@enea.com>