<property name="locations">
  <value>file:${activemq.base}/conf/credentials.properties</value>
</property>

jetty.xml:

<bean id="securityLoginService" class="org.eclipse.jetty.security.HashLoginService">
  <property name="name" value="ActiveMQRealm" />
  <property name="config" value="${activemq.base}/conf/jetty-realm.properties" />
</bean>

webconsole-embedded.xml:

<property name="locations">
  <value>file:${activemq.base}/conf/credentials.properties</value>
</property>

and others.
In case where you want to externalize the entire ActiveMQ configuration into a folder that is not a sub folder of AMQ, there is the need to change multiple configuration files. This is error prone.

In order to make the use of external AMQ configuration more easy to use, I suggest to
1. introduce a new property called activemq.conf, which points to ${activemq.base}/conf by default in activemq script (if not already set on the shell)
2. replace all occurences of ${activemq.base}/conf with {$activemq.conf} in all XML configuration files (including those in webapps/ folder)

That way one only needs to set the property activemq.conf in order to point to an external AMQ configuration folder.