Organize OpenPOWER BMC support as features

Add OBMC_MACHINE_FEATURES for OpenPOWER support under openpower.inc.
Select OpenPOWER features in the POWER machines' DISTRO_FEATURES so that
the features are built.

The features are:
  obmc-openpower for required base support
  obmc-openpower-extra for non-essential tools
  obmc-openpower-legacy for deprecated packages like org.openbmc and
    skeleton packages

Part of openbmc/openbmc#2652.

Change-Id: Ib9f44a0c2e243eebb6594de6554cdc3fa0d076af
Signed-off-by: Xo Wang <xow@google.com>
diff --git a/meta-garrison/conf/machine/garrison.conf b/meta-garrison/conf/machine/garrison.conf
index 436c68a..089c312 100644
--- a/meta-garrison/conf/machine/garrison.conf
+++ b/meta-garrison/conf/machine/garrison.conf
@@ -11,4 +11,10 @@
 
 FLASH_SIZE = "32768"
 
+DISTRO_FEATURES_append = "\
+        obmc-openpower \
+        obmc-openpower-extra \
+        obmc-openpower-legacy \
+        "
+
 PREFERRED_PROVIDER_virtual/phosphor-ipmi-fru-inventory = "garrison-ipmi-inventory-map-native"