The switch_root shell script did not work with bash-4.0-2, because
"exec" gets the real path of the executable which is then
"/sysroot/lib/ld-linux.so.2" instread of "./lib/ld-linux.so.2".

Also the required chroot binary might live in /usr/bin, which can
be mounted later.

Here is the switch_root code from nash, which can be stripped down
further, but which works.