version.sh: Fix in-tree case and allow external variables

"$(basename $0)" would return the name of the script, when what is
needed is the name of the directory; use 'dirname' instead.
If PETITBOOT_VERSION is set, as in a buildroot environment, use this
variable instead of the default case.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>