feeds: add freifunk feed

Read the freifunk packages, that have been moved from the LuCI feed
into its own feed in January 2019.
Use openwrt-19.07 branch of that repository for openwrt-19.07.

Signed-off-by: Sven Roederer <freifunk@it-solutions.geroedel.de>
(cherry picked from commit 221f97ff4737f012c90feb086bc1c2ed86c6001b)
Signed-off-by: Daniel Golle <daniel@makrotopia.org>

feeds.conf.default

@@ -2,3 +2,4 @@ src-git packages https://git.openwrt.org/feed/packages.git;openwrt-19.07
 src-git luci https://git.openwrt.org/project/luci.git;openwrt-19.07
 src-git routing https://git.openwrt.org/feed/routing.git;openwrt-19.07
 src-git telephony https://git.openwrt.org/feed/telephony.git;openwrt-19.07
+src-git freifunk https://github.com/freifunk/openwrt-packages.git;openwrt-19.07