u-boot-script-boundary: Use bb vars for overrides

When using bitbake variables, overrides in bbappends are possible.
The embedded shell variables did not allow this.

Signed-off-by: Christoph Settgast <christoph.settgast@methodpark.de>
Reviewed-by: Eric Nelson <eric.nelson@boundarydevices.com>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>