[nrf temphack] tfm: Kconfig: Disable TFM_BL2 when using CryptoCell

There is a regression in nrf_security with regards to building BL2
and Crypto Services making the BL2 non-functional.
This will be resolved later.

Note: this commit reapplies da17ae6
      which was lost in another commit.

Signed-off-by: Øyvind Rønningstad <[email protected]>
(cherry picked from commit bfd1376)

Author: oyvindronningstad

modules/trusted-firmware-m/Kconfig.tfm

@@ -142,9 +142,12 @@ config TFM_ISOLATION_LEVEL
 config TFM_BL2
 	bool "Add MCUboot to TFM"
 	default y
+	depends on !NORDIC_SECURITY_BACKEND
 	help
 	  TFM is designed to run with MCUboot in a certain configuration.
 	  This config adds MCUboot to the build - built via TFM's build system.
+	  We currently do not support builds with MCUboot and TF-M if the
+	  Nordic Security backend is used.
 
 config TFM_MCUBOOT_IMAGE_NUMBER
 	int "Granularity of FW updates of TFM and app"