zephyr: Kconfig: fix board references

Upstream Zephyr has renamed the 'nonsecure' variants for boards that
support TF-M. The rename consistently uses '_ns' or '_NS' (with
underscore) as the postfix which distinguishes these variants from
their 'secure' versions.

This affects Kconfig symbols used by MCUboot, so fix them to keep
working with the latest zephyr main branch.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>