ipkg-utils*: Re-add support for openwrt-style .ipk's.

ipkg-utils*: Re-add support for openwrt-style .ipk's.
* There're two .ipk formats: one with ar top-level wrapping (essentially, .deb
format, native OE format) and with tar.gz top-level wrap, used by OpenWRT and some
tiny distros. When I did performance optimization work on ipkg-make-index, I just
blocked support of tar.gz-style ipk's, as it was unclear if those are used in OE
at all. Now support is readded, with the old code to handle them (i.e. not as optimal
as ar-style ones).
* This makes DISTRO=openwrt-sdk usable for producing compatible packages and
deploy-feed for them. But there're still some issues to resolve before OE can be
said to provide seamless support for building packages for OpenWRT.