Build: Unify isolation level setting with TFM_ISOLATION_LEVEL

There is a CMake configuration "TFM_ISOLATION_LEVEL", which can also be
used as the CMake compile definition. To make the code easier to
maintain, remove current compile definition "TFM_LVL" and unify the
isolation level setting with "TFM_ISOLATION_LEVEL".

Signed-off-by: Xinyu Zhang <xinyu.zhang@arm.com>
Change-Id: I659744b470e8da6f14457094402e45cf77964406
diff --git a/secure_fw/partitions/lib/runtime/service_api.c b/secure_fw/partitions/lib/runtime/service_api.c
index 1ed1ad9..3e9576d 100644
--- a/secure_fw/partitions/lib/runtime/service_api.c
+++ b/secure_fw/partitions/lib/runtime/service_api.c
@@ -22,7 +22,7 @@
         );
 }
 
-#if TFM_LVL != 1
+#if TFM_ISOLATION_LEVEL != 1
 /* Entry point when Partition FLIH functions return */
 __attribute__((naked))
 void tfm_flih_func_return(psa_flih_result_t result)
@@ -30,4 +30,4 @@
     __ASM volatile("SVC "M2S(TFM_SVC_FLIH_FUNC_RETURN)"           \n"
                    );
 }
-#endif /* TFM_LVL != 1 */
+#endif /* TFM_ISOLATION_LEVEL != 1 */