meta-phosphor: allow overriding default virtual/obmc-system-mgmt

Allow another distro building from OpenBMC to specify their own default
virtual/obmc-system-mgmt to allow the meta-ingrasys/meta-openpower
distro-feature virtual/obmc-system-mgmt to set within a system providing
a different default.

Tested: Zaius image built with correct openpower overrides.  Other
platforms built with expected value.
Signed-off-by: Patrick Venture <venture@google.com>
Change-Id: Ia668bdd94bda921dd4fa2d7e8c849a4a4f3677f9
diff --git a/conf/distro/include/phosphor-defaults.inc b/conf/distro/include/phosphor-defaults.inc
index 0128fce..2fed684 100644
--- a/conf/distro/include/phosphor-defaults.inc
+++ b/conf/distro/include/phosphor-defaults.inc
@@ -102,7 +102,7 @@
 # Provide support for monitoring BMC core dumps
 VIRTUAL-RUNTIME_obmc-dump-monitor ?= "phosphor-debug-collector-monitor"
 
-PREFERRED_PROVIDER_virtual/obmc-system-mgmt ?= "obmc-phosphor-sysd"
+PREFERRED_PROVIDER_virtual/obmc-system-mgmt ??= "obmc-phosphor-sysd"
 
 # phosphor-hwmon-config
 #