carl9170 firmware build: mkdir include/generated when not present

Quote from Marius L. Meyer:
I discovered what the problem was. The directory include/generated does not
exist and does not get created anywhere in the process. So when
configure/conf tries to rename .tmpconfig.h to include/generated/autoconf.h,
it fails. The problem is solved by simply creating the directory before
running autogen.sh.

Reported-by: Marius Ladegaard Meyer
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>