Allow build_chroot to work from any directory

build_chroot used to work only if launched from ./devel/, whereas now
it can be launched from anywhere, and it will store the resulting files
in the current directory.

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>