meta-phosphor: ipmid: enable EXTRA_OECONF override

Set the whitelist with the post-parse append operator, so ipmid
bbappends can set EXTRA_OECONF directly.

A simple example illustrates how this helps:
 foo.bb:
  BAR = "def"
  FOO = "abc ${BAR}
 foo.bbappend:
  FOO = "zyx"

FOO would be "zyx"  But if one has:
 foo.bb:
  BAR = "def"
  FOO = "abc"
  FOO_append = " ${BAR}"
 foo.bbappend:
  FOO = "zyx"

FOO would be "zyx def"

This is because bitbake parses recipes and bbappends in multiple
passes - the second pass is when things like the _append operator
and inline python are evaluated.

for more reading:

https://www.yoctoproject.org/docs/latest/bitbake-user-manual/bitbake-user-manual.html#override-style-operation-advantages

(From meta-phosphor rev: bb60cffd406a8a413be0ce70b65d9b3447c95bbf)

Change-Id: I2c9a4eaf65c815ebdf18ff158de75448837872b4
Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>
diff --git a/meta-phosphor/recipes-phosphor/ipmi/phosphor-ipmi-host_git.bb b/meta-phosphor/recipes-phosphor/ipmi/phosphor-ipmi-host_git.bb
index 1438eaa..314b70f 100644
--- a/meta-phosphor/recipes-phosphor/ipmi/phosphor-ipmi-host_git.bb
+++ b/meta-phosphor/recipes-phosphor/ipmi/phosphor-ipmi-host_git.bb
@@ -71,12 +71,14 @@
         ${S}/host-ipmid-whitelist.conf \
         "
 EXTRA_OECONF = " \
-        WHITELIST_CONF="${WHITELIST_CONF}" \
         SENSOR_YAML_GEN=${STAGING_DIR_NATIVE}${sensor_datadir}/sensor.yaml \
         INVSENSOR_YAML_GEN=${STAGING_DIR_NATIVE}${sensor_datadir}/invsensor.yaml \
         FRU_YAML_GEN=${STAGING_DIR_NATIVE}${config_datadir}/fru_config.yaml \
         ENTITY_YAML_GEN=${STAGING_DIR_NATIVE}${sensor_datadir}/entity.yaml \
         "
+EXTRA_OECONF_append = " \
+        WHITELIST_CONF="${WHITELIST_CONF}" \
+        "
 
 S = "${WORKDIR}/git"