Use webpack instead of rollup for the legacy build

This also changes the target path of the legacy build to build/legacy

config/rollup.js

@@ -1,21 +0,0 @@
-// Rollup configuration for the full build
-
-import noderesolve from 'rollup-plugin-node-resolve';
-import commonjs from 'rollup-plugin-commonjs';
-import {uglify} from 'rollup-plugin-uglify';
-import buble from 'rollup-plugin-buble';
-import sourcemaps from 'rollup-plugin-sourcemaps';
-
-export default {
-  input: 'build/index.js',
-  output: [
-    {file: 'build/ol.js', format: 'iife', sourcemap: true}
-  ],
-  plugins: [
-    noderesolve(),
-    commonjs(),
-    buble(),
-    uglify(),
-    sourcemaps()
-  ]
-};

config/webpack-config-legacy-build.js

@@ -0,0 +1,13 @@
+const path = require('path');
+module.exports = {
+  entry: './build/index.js',
+  devtool: 'source-map',
+  mode: 'production',
+  output: {
+    path: path.resolve('./build/legacy'),
+    filename: 'ol.js',
+    library: 'ol',
+    libraryTarget: 'umd',
+    libraryExport: 'default'
+  }
+};