The problem is that you need ros binaries to build ros itself... so you must
keep a cross-compiled version and a version of the architecture you are
using to build ros.

You may have a look there:
http://dave.limoilux.org/trac/wiki/ROS/CrossCompile/arm-2

I explain how I cross-compile ROS for ARM using the ROS build scripts.

-- 
Benjamin Vanheuverzwijn

Google Talk/Jabber - bvanheu@gmail.com
http://vanheu.ca