base-files: add missing public key

This patch is already included in the current OpenWrt CC branch.
Cherry-pick for now as we're very close to the Gluon 2015.2 release.

patches/openwrt/0041-CC-base-files-add-missing-public-key.patch

+From: Matthias Schiffer <mschiffer@universe-factory.net>
+Date: Sat, 21 Nov 2015 18:35:49 +0100
+Subject: CC: base-files: add missing public key
+
+Add the secondary package signature key to the 15.05 sources so that people
+building the release have the same keys as people using precompiled images.
+
+Signed-off-by: Jo-Philipp Wich <jow@openwrt.org>
+
+diff --git a/package/base-files/files/etc/opkg/keys/de98a2dd1d0f8a07 b/package/base-files/files/etc/opkg/keys/de98a2dd1d0f8a07
+new file mode 100644
+index 0000000..5e3088c
+--- /dev/null
++++ b/package/base-files/files/etc/opkg/keys/de98a2dd1d0f8a07
+@@ -0,0 +1,2 @@
++untrusted comment: openwrt.org 15.05 release key
++RWTemKLdHQ+KBxOILy8gyk+5PaDVdfyJ32TFnY/jnQOrBAd1wobbLNYz