meta-ibm: Using append in override syntax

Currently, scripts in install_ibm_plugins link_ibm_plugins are
not getting added in BMC image as a result of which changing "+="
to append.

Tested:
Post change can see the scripts are getting added in the image.

Change-Id: I932257e3652d0d6911923d4e9e0a57abeeb392ce
Signed-off-by: Chirag Sharma <chirshar@in.ibm.com>
diff --git a/meta-ibm/recipes-phosphor/dump/phosphor-debug-collector_%.bbappend b/meta-ibm/recipes-phosphor/dump/phosphor-debug-collector_%.bbappend
index e9ae74b..df135f4 100644
--- a/meta-ibm/recipes-phosphor/dump/phosphor-debug-collector_%.bbappend
+++ b/meta-ibm/recipes-phosphor/dump/phosphor-debug-collector_%.bbappend
@@ -33,6 +33,6 @@
 }
 
 IBM_INSTALL_POSTFUNCS = "install_ibm_plugins link_ibm_plugins"
-IBM_INSTALL_POSTFUNCS:p10bmc += "install_dreport_header"
+IBM_INSTALL_POSTFUNCS:append:p10bmc = " install_dreport_header"
 
 do_install[postfuncs] += "${IBM_INSTALL_POSTFUNCS}"