stop overriding DISTRO_FEATURES in phosphor-base

Other include files may set DISTRO_FEATURES prior to including this
file. Need to honor those features.

(From meta-phosphor rev: 17801b40a7143c6b80dd4318c94143e47ef1ead3)

Change-Id: I42593e07d67416d2dbb06f3958861ed4ebe762ff
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-phosphor/conf/distro/include/phosphor-base.inc b/meta-phosphor/conf/distro/include/phosphor-base.inc
index 2a9301d..20fd216 100644
--- a/meta-phosphor/conf/distro/include/phosphor-base.inc
+++ b/meta-phosphor/conf/distro/include/phosphor-base.inc
@@ -113,7 +113,7 @@
         libc-inet-anl \
         "
 
-DISTRO_FEATURES = " \
+DISTRO_FEATURES += " \
         ${DISTRO_FEATURES_DEFAULT} \
         ${DISTRO_FEATURES_LIBC} \
         ${DISTRO_FEATURES_OBMC} \