package_manager: ipk: add OPKG_MAKE_INDEX_EXTRA_PARAMS variable

package_manager: ipk: add OPKG_MAKE_INDEX_EXTRA_PARAMS variable

* can be used to pass e.g. -f param to preserve user-defined fields
  in the index as added in:
  https://git.yoctoproject.org/opkg-utils/commit/opkg-make-index?id=13f6281d24e17199e0fef6c2984419372ea0f86f

* otherwise it will show a lot of messages like:
  "Lost field Author <value>"
  for every package in the feed

Signed-off-by: Martin Jansa <martin.jansa@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>