My working gruntfile.

Gruntfile.js

module.exports = function(grunt) {

    require('time-grunt')(grunt);

    require('load-grunt-config')(grunt, {
        jitGrunt: true
    });

    grunt.initConfig({
        less: {
            development: {
                options: {
                    compress: false,
                    yuicompress: false,
                    optimization: 2,
                    sourceMap: true,
                    // sourceMapFilename: "assets/style/bootstrap.css.map",
                    sourceMapFilename: "style.css.map",
                    // sourceMapBasepath: "assets/style/"
                    sourceMapBasepath: "html/sites/all/themes/corvilla/css/"
                },
                files: {
                    // target.css file: source.less file
                    // "assets/style/bootstrap.css": "assets/style/bootstrap.less"
                    "html/sites/all/themes/corvilla/css/style.css" : "html/sites/all/themes/corvilla/less/style.less"
                }
            }
        },
        watch: {
            styles: {
                // Which files to watch (all .less files recursively in the less directory)
                // files: ['assets/style/theme/**/*.less'],
                files: ['html/sites/all/themes/corvilla/**/*.less'],
                tasks: ['less'],
                options: {
                    nospawn: true
                }
            }
        }
    });

    grunt.loadNpmTasks('grunt-contrib-less');
    grunt.loadNpmTasks('grunt-contrib-watch');

    grunt.registerTask('default', ['watch']);
};