Custom rootfs support

This fixes Issue #43

The custom rootfs support is command line/env only as you can hardly expect a human to correctly enter a 40-characted file name + path to it.

The configure script will still ask for arch/variant/subarch though it will be ignored.

best used with -q

hopefully the README should display correctly (I escaped _ in $STAGE3_ROOTFS with \ because vim just decided to highlight the rest of the script in bold white).

Note that while it is possible now for this script to generate rootfss for different architectures, there still is no easy way to run them.