Create a blank era for sp

Also, moved the "Custom" mp side to be loaded outside of the MULTIPLAYER define.

data/_main.cfg

@@ -17,10 +17,17 @@
 
 {core/}
 
-{multiplayer/eras.cfg}
+[multiplayer_side]
+    id=Custom
+    name= _"Custom"
+    image="units/unknown-unit.png"
+    {MAGENTA_IS_THE_TEAM_COLOR}
+[/multiplayer_side]
 
 #ifdef MULTIPLAYER
 {multiplayer/}
+#else
+{era_blank.cfg}
 #endif
 
 {campaigns/}

data/era_blank.cfg

@@ -0,0 +1,6 @@
+#textdomain wesnoth
+[era]
+    id=era_blank
+    name= _ "No era"
+    description=_ "An empty era to be used for single player campaigns."
+[/era]

data/multiplayer/eras.cfg

@@ -1,12 +1,5 @@
 #textdomain wesnoth-multiplayer
 
-[multiplayer_side]
-    id=Custom
-    name= _"Custom"
-    image="units/unknown-unit.png"
-    {MAGENTA_IS_THE_TEAM_COLOR}
-[/multiplayer_side]
-
 #define RANDOM_SIDE
     [multiplayer_side]
         id=Random