trustfence: fix trustfence support in NXP platforms

In commit df9b1cf329, the UBOOT_SIGN_ENABLE is set for all
platforms, and should be only added for FIT images.

This is making the process failing in cc8mn/cc8mm platforms
due to the UBOOT_SIGN_ENABLE is also used there to use a dtb
patched with the signature node.

https://onedigi.atlassian.net/browse/DEL-8764

Signed-off-by: Francisco Gil francisco.gilmartinez@digi.com

meta-digi-dey/classes/trustfence.bbclass

@@ -192,6 +192,8 @@ python () {
             if d.getVar("TRUSTFENCE_SIGN_MODE"):
                 d.appendVar("UBOOT_TF_CONF", 'CONFIG_SIGN_MODE="%s" ' % d.getVar("TRUSTFENCE_SIGN_MODE"))
 
+
+        if (d.getVar("TRUSTFENCE_FIT_IMG") == "1"):
             # FIT-related variables
             # Create keys if not defined
             d.setVar("FIT_GENERATE_KEYS", "1")