Ensure that a build dir exists for the recipe. This same single
        dir will be used for building all different archs.

def prepare_build_dir(self):
        '''Ensure that a build dir exists for the recipe. This same single
        dir will be used for building all different archs.'''
        self.build_dir = self.get_build_dir()
        self.common_dir = self.get_common_dir()
        copy_files(join(self.bootstrap_dir, 'build'), self.build_dir)
        copy_files(join(self.common_dir, 'build'), self.build_dir,
                   override=False)
        if self.ctx.symlink_java_src:
            info('Symlinking java src instead of copying')
            shprint(sh.rm, '-r', join(self.build_dir, 'src'))
            shprint(sh.mkdir, join(self.build_dir, 'src'))
            for dirn in listdir(join(self.bootstrap_dir, 'build', 'src')):
                shprint(sh.ln, '-s', join(self.bootstrap_dir, 'build', 'src', dirn),
                        join(self.build_dir, 'src'))
        with current_directory(self.build_dir):
            with open('project.properties', 'w') as fileh:
                fileh.write('target=android-{}'.format(self.ctx.android_api))