meta-google: Set entiy-map to `[]` by default for gBMC

The entity map is now used for ipmid/dbus-sdr and will cause ipmi
failures after
https://gerrit.openbmc-project.xyz/c/openbmc/phosphor-host-ipmid/+/46882

Remove the default json and only allow system that require it to
override it.

Tested:
```
$ cat ./tmp/work/all-openbmc-linux/phosphor-ipmi-config/1.0-r1/package/usr/share/ipmi-providers/entity-map.json
[]
```

Change-Id: I0ef8248a0d7d99fa0c9e3199c4eceb664bc624cf
Signed-off-by: Willy Tu <wltu@google.com>
diff --git a/meta-google/recipes-phosphor/ipmi/phosphor-ipmi-config.bbappend b/meta-google/recipes-phosphor/ipmi/phosphor-ipmi-config.bbappend
index 56f7933..aa2e266 100644
--- a/meta-google/recipes-phosphor/ipmi/phosphor-ipmi-config.bbappend
+++ b/meta-google/recipes-phosphor/ipmi/phosphor-ipmi-config.bbappend
@@ -18,5 +18,9 @@
   jq --slurpfile brcfg ${WORKDIR}/gbmc_bridge.json \
     '. + {"${GBMCBR_IPMI_CHANNEL}": $brcfg[0]}' $chjson >${WORKDIR}/tmp
   mv ${WORKDIR}/tmp $chjson
+
+  # Set entity-map.json to empty json for gBMC by default.
+  # Each system will override it if needed.
+  echo "[]" > ${D}${datadir}/ipmi-providers/entity-map.json
 }