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 use 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).