build_chroot: check whether the data dir exists

If the data directory is not in the expected place, the script complains
with an error message and stops, instead of giving obscure messages.

Partially fixes Issue 459.

Signed-off-by: Michele Tartara <mtartara@google.com>
Reviewed-by: Klaus Aehlig <aehlig@google.com>
Reviewed-by: Guido Trotter <ultrotter@google.com>