samples: tfm_ipc: add test-variant for TF-M without BL2

We add a test variant in tfm_ipc test, to validate the
scenario where TF-M is built without BL2 (MCUboot). We
test this on QEMU only (MPS2 AN521).

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>