powermgmt: Small cleanups to battery capacity code

- Don't include the 'battery capacity' setting unless the
  target allows changing it.
- Clean up the preprocessor conditionals used to check for
  variable battery capacity support.
- Don't use a variable for battery capacity unless it is
  actually needed.

Change-Id: I3d8a338f107014f2c5098bc0a44ef0cfb4df9356

firmware/export/config.h

@@ -709,6 +709,14 @@ Lyre prototype 1 */
 #define BATTERY_CAPACITY_DEFAULT 0
 #endif
 
+#ifndef BATTERY_CAPACITY_MIN
+#define BATTERY_CAPACITY_MIN BATTERY_CAPACITY_DEFAULT
+#endif
+
+#ifndef BATTERY_CAPACITY_MAX
+#define BATTERY_CAPACITY_MAX BATTERY_CAPACITY_DEFAULT
+#endif
+
 #ifndef BATTERY_CAPACITY_INC
 #define BATTERY_CAPACITY_INC 0
 #endif