include/feeds.mk: fix distfeeds.conf without per-feed repos

include/feeds.mk: fix distfeeds.conf without per-feed repos

commit 514a4b3e1b4e4 ("include/feeds.mk: rework generation of opkg
distfeeds.conf") made the per-feed "base" repo unconditional, making
the default configuration fail when PER_FEED_REPO is disabled:

 root@wrt1900ac-1:~# cat /etc/opkg/distfeeds.conf
 src/gz openwrt_core http://openwrt.mork.no/18.06.0/targets/mvebu/cortexa9/packages
 src/gz openwrt_base http://openwrt.mork.no/18.06.0/packages/arm_cortex-a9_vfpv3/base
 root@wrt1900ac-1:~# opkg update
 Downloading http://openwrt.mork.no/18.06.0/targets/mvebu/cortexa9/packages/Packages.gz
 Updated list of available packages in /var/opkg-lists/openwrt_core
 Downloading http://openwrt.mork.no/18.06.0/targets/mvebu/cortexa9/packages/Packages.sig
 Signature check passed.
 Downloading http://openwrt.mork.no/18.06.0/packages/arm_cortex-a9_vfpv3/base/Packages.gz
 *** Failed to download the package list from http://openwrt.mork.no/18.06.0/packages/arm_cortex-a9_vfpv3/base/Packages.gz

 Collected errors:
  * opkg_download: Failed to download http://openwrt.mork.no/18.06.0/packages/arm_cortex-a9_vfpv3/base/Packages.gz, wget returned 8.

Cc: Matthias Schiffer <mschiffer@universe-factory.net>
Fixes: 514a4b3e1b4e ("include/feeds.mk: rework generation of opkg distfeeds.conf")
Signed-off-by: Bjørn Mork <bjorn@mork.no>
[whitespace/indentation fix]
Signed-off-by: Jo-Philipp Wich <jo@mein.io>