phosphor-tiny: preserve existing distro overrides

phosphor-tiny.inc is intended to be included from a distro config file.
As such it should not override any existing content in DISTROOVERRIDES
placed there by other distro includes or the main distro configuration
file.

Fixes: 8ed79d3c02f088673a52e89168f3d44b5a736ab0
Change-Id: Ia71b91f71140ecbc954df84060d9e9d57b36b648
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/conf/distro/include/phosphor-tiny.inc b/conf/distro/include/phosphor-tiny.inc
index 8d57b54..990b44e 100644
--- a/conf/distro/include/phosphor-tiny.inc
+++ b/conf/distro/include/phosphor-tiny.inc
@@ -1,2 +1,2 @@
 # Include this to get minimum flash image created
-DISTROOVERRIDES = "openbmc-phosphor:openbmc-phosphor-tiny"
+DISTROOVERRIDES .= ":openbmc-phosphor-tiny"