Merge pull request #12326 from twbs/navbar-var

add @navbar-collapse-max-height Less variable

less/navbar.less

@@ -48,7 +48,7 @@
 // content for the user's viewport.
 
 .navbar-collapse {
-  max-height: 340px;
+  max-height: @navbar-collapse-max-height;
   overflow-x: visible;
   padding-right: @navbar-padding-horizontal;
   padding-left:  @navbar-padding-horizontal;

less/variables.less

@@ -311,6 +311,7 @@
 @navbar-border-radius:             @border-radius-base;
 @navbar-padding-horizontal:        floor((@grid-gutter-width / 2));
 @navbar-padding-vertical:          ((@navbar-height - @line-height-computed) / 2);
+@navbar-collapse-max-height:       340px;
 
 @navbar-default-color:             #777;
 @navbar-default-bg:                #f8f8f8;