include/feeds.mk: always add available feeds to PACKAGE_SUBDIRS

include/feeds.mk: always add available feeds to PACKAGE_SUBDIRS

Setting CONFIG_FEED_... symbols combined two different effects: Disabling
a feed in the generated opkg distfeeds.conf, and omitting the feed from
PACKAGE_SUBDIRS.

It does not make sense to omit built feeds from PACKAGE_SUBDIRS, as it will
only lead to packages that can be enabled in .config (and that will
consequently be built) not to be found during rootfs creation, breaking
the build. All feeds that packages are emitted to should simply always be
added to PACKAGE_SUBDIRS instead; the CONFIG_FEED_... only configure the
generated distfeeds.conf like this.

Signed-off-by: Matthias Schiffer <mschiffer@universe-factory.net>