stop overriding DISTRO_FEATURES in phosphor-base

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

Change-Id: I67eb80b41f41ab40db85db26a8b7ef6634e4b0c3
Signed-off-by: Andrew Geissler <geissonator@yahoo.com>
diff --git a/conf/distro/include/phosphor-base.inc b/conf/distro/include/phosphor-base.inc
index 2a9301d..20fd216 100644
--- a/conf/distro/include/phosphor-base.inc
+++ b/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} \