Use idmap_ignore for optional mapping files

These mapping files are only needed in certain configurations. We need
to use idmap_ignore so that the driver's buildnml won't abort for
configurations where these mapping files aren't provided and aren't
needed.

src/drivers/mct/cime_config/config_component.xml

@@ -1628,10 +1628,11 @@
 
   <entry id="GLC2ICE_FMAPNAME">
     <type>char</type>
-    <default_value>idmap</default_value>
+    <default_value>idmap_ignore</default_value>
     <group>run_domain</group>
     <file>env_run.xml</file>
-    <desc>glc2ice flux mapping file</desc>
+    <desc>glc2ice flux mapping file - the default value idmap_ignore, if set, will be ignored by buildnml and
+    will generate a runtime error if in fact a file is required for the given compset</desc>
   </entry>
 
   <entry id="GLC2ICE_FMAPTYPE">
@@ -1645,10 +1646,11 @@
 
   <entry id="GLC2ICE_SMAPNAME">
     <type>char</type>
-    <default_value>idmap</default_value>
+    <default_value>idmap_ignore</default_value>
     <group>run_domain</group>
     <file>env_run.xml</file>
-    <desc>glc2ice state mapping file</desc>
+    <desc>glc2ice state mapping file - the default value idmap_ignore, if set, will be ignored by buildnml and
+    will generate a runtime error if in fact a file is required for the given compset</desc>
   </entry>
 
   <entry id="GLC2ICE_SMAPTYPE">
@@ -1680,10 +1682,11 @@
 
   <entry id="GLC2OCN_FMAPNAME">
     <type>char</type>
-    <default_value>idmap</default_value>
+    <default_value>idmap_ignore</default_value>
     <group>run_domain</group>
     <file>env_run.xml</file>
-    <desc>glc2ocn flux mapping file</desc>
+    <desc>glc2ocn flux mapping file - the default value idmap_ignore, if set, will be ignored by buildnml and
+    will generate a runtime error if in fact a file is required for the given compset</desc>
   </entry>
 
   <entry id="GLC2OCN_FMAPTYPE">
@@ -1697,10 +1700,11 @@
 
   <entry id="GLC2OCN_SMAPNAME">
     <type>char</type>
-    <default_value>idmap</default_value>
+    <default_value>idmap_ignore</default_value>
     <group>run_domain</group>
     <file>env_run.xml</file>
-    <desc>glc2ocn state mapping file</desc>
+    <desc>glc2ocn state mapping file - the default value idmap_ignore, if set, will be ignored by buildnml and
+    will generate a runtime error if in fact a file is required for the given compset</desc>
   </entry>
 
   <entry id="GLC2OCN_SMAPTYPE">
@@ -1750,10 +1754,11 @@
 
   <entry id="OCN2GLC_FMAPNAME">
     <type>char</type>
-    <default_value>idmap</default_value>
+    <default_value>idmap_ignore</default_value>
     <group>run_domain</group>
     <file>env_run.xml</file>
-    <desc>ocn2glc flux mapping file</desc>
+    <desc>ocn2glc flux mapping file - the default value idmap_ignore, if set, will be ignored by buildnml and
+    will generate a runtime error if in fact a file is required for the given compset</desc>
   </entry>
 
   <entry id="OCN2GLC_FMAPTYPE">
@@ -1767,10 +1772,11 @@
 
   <entry id="OCN2GLC_SMAPNAME">
     <type>char</type>
-    <default_value>idmap</default_value>
+    <default_value>idmap_ignore</default_value>
     <group>run_domain</group>
     <file>env_run.xml</file>
-    <desc>ocn2glc state mapping file</desc>
+    <desc>ocn2glc state mapping file - the default value idmap_ignore, if set, will be ignored by buildnml and
+    will generate a runtime error if in fact a file is required for the given compset</desc>
   </entry>
 
   <entry id="OCN2GLC_SMAPTYPE">