modules: tfm: fix `TFM_MCUBOOT_IMAGE_NUMBER == 1`

Fix builds with `CONFIG_TFM_MCUBOOT_IMAGE_NUMBER=1`. The merged binary
should be signed with the same argumements as a secure build, not a
non-secure build, except with a different layout file generated by the
TF-M build system.

Fixes #68345.

Signed-off-by: Jordan Yates <jordan@embeint.com>