scripts: add tool for generating redboot scripts

This tool avoids generating the redboot script directly from Makefile, which
can get really complicated when adapting image/Makefile to the new IB system.

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Jonas Gorski <jogo@openwrt.org>

SVN-Revision: 46488