package_manager.py: fix python indentation bug (opkg)

package_manager.py: fix python indentation bug (opkg)

This if branch is causing the following error during do_rootfs:
"Exception: UnboundLocalError: local variable 'cfg_file' referenced
before assignment". This happends because the cfg_file variable is
defined at a deeper nesting level (just above the if branch).

Signed-off-by: Ioan-Adrian Ratiu <adrian.ratiu@ni.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>