fwu: Fix direct_xip builds for NS side

MCUBOOT_EXECUTION_SLOT is handled by cmake and
the platform headers. To enable building NS side
in the secondary slot, we have to pass this
definition.

Note:
Add -DMCUBOOT_EXECUTION_SLOT=2 to the S AND NS build too
if you want to build for secondary slot.

Signed-off-by: Dávid Házi <david.hazi@arm.com>
Change-Id: I5cbf069ec3faca54bcc702306fefece13f0b6aa5