Build: Fix MCUBOOT_KEY_S/NS export value

MCUBOOT_KEY_S/NS values vary in different cases and therefore export
their runtime values to NS, rather the default ones.

Change-Id: If0e5086ce8a47ac25d70f17ade18f2bf798c994f
Signed-off-by: David Hu <david.hu@arm.com>
2 files changed