Building

http://source.android.com/source/download.html
JDK 5.0, update 12 or higher.Java 6 is not supported, because of incompatibilities with @Override.

This needs properly writing, the following is for reference only…

mkdir froyo && cd froyo && repo init -u git://github.com/iDroid-Project/platform_manifest.git -b froyo-iphone
repo sync
. build/envsetup.sh && choosecombo

—> device (2 iirc), release or debug (depends what you want), iPhone3G or iPhone2G or iPodTouch1G (depends what you want), eng
make -j2 PRODUCT-iPhone3G-eng

iPhone3G replaceable by iPhone2G, iPodTouch1G,.. you know
(-j = num of compile processes, more if multicore, 2 per core)

if you want firmware in /private/var/firmware/
you need to mount the system img, erase the /etc/firmware in it, symling the /etc/firmware/ in it to /host/firmware/