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