phosphor-base: fix DISTRO_FEATURES_DEFAULTS variable collision with OE-core

phosphor-base.inc redefined DISTRO_FEATURES_DEFAULTS and then
explicitly appended it to DISTRO_FEATURES. DISTRO_FEATURES_DEFAULTS
is owned by OE-core (meta/conf/distro/include/default-distrovars.inc)
where it is defined with a weak ?= assignment and automatically
seeded into DISTRO_FEATURES during BitBake initialization.

Redefining it caused avahi, ipv4, ipv6, pam, security, and seccomp to
appear twice in the final DISTRO_FEATURES expansion,
once from OE-core's automatic seeding and once from the
explicit += in phosphor-base.inc.

Fix this by splitting the variable usage into two roles:

1. DISTRO_FEATURES_DEFAULTS: intentionally overrides OE-core's weak ?=
   to a BMC-appropriate subset (ipv4, ipv6, seccomp), suppressing
   OE-core features irrelevant to a BMC (bluetooth, wifi, x11, etc).
   OE-core seeds this into DISTRO_FEATURES automatically — do not
   append it again explicitly.

2. DISTRO_FEATURES_PHOSPHOR_DEFAULTS: new variable holding phosphor-
   specific features not present in OE-core defaults (avahi, pam,
   security). This is appended explicitly to DISTRO_FEATURES.

Verified: zero duplicate entries in DISTRO_FEATURES after this change.
Full phosphor feature set preserved. No unintended OE-core features
introduced.

Change-Id: I7a3d258f1875af004fb0165bb144af2046fcc563
Signed-off-by: Vijaysankar Ravi <vijaysankarr@ami.com>

meta-phosphor/conf/distro/include/phosphor-base.inc

diff --git a/meta-phosphor/conf/distro/include/phosphor-base.inc b/meta-phosphor/conf/distro/include/phosphor-base.inc
index a77ffbd..df1d1dd 100644
--- a/meta-phosphor/conf/distro/include/phosphor-base.inc
+++ b/meta-phosphor/conf/distro/include/phosphor-base.inc
@@ -102,17 +102,25 @@
         obmc-phosphor-system-mgmt \
         "
 
+# BMC-appropriate override of OE-core's DISTRO_FEATURES_DEFAULTS (weak ?=
+# in default-distrovars.inc, which also seeds bluetooth/wifi/x11/etc into
+# DISTRO_FEATURES). Already seeded automatically by OE-core — do not
+# append below or it will duplicate entries.
 DISTRO_FEATURES_DEFAULTS = " \
-        avahi \
         ipv4 \
         ipv6 \
-        pam \
-        security \
         seccomp \
         "
 
+# Phosphor-specific features not present in OE-core defaults.
+DISTRO_FEATURES_PHOSPHOR_DEFAULTS = " \
+        avahi \
+        pam \
+        security \
+        "
+
 DISTRO_FEATURES += " \
-        ${DISTRO_FEATURES_DEFAULTS} \
+        ${DISTRO_FEATURES_PHOSPHOR_DEFAULTS} \
         ${DISTRO_FEATURES_OBMC} \
         "