Adjust prompt for the post-build scripts option

Since the post-image config option uses the plural for "Custom
scripts", do the same for the post-build config option, for
consistency.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>