Add psa_trusted_storage_linux persistent storage support for v1.0.0 APIs

The following provides more information on this PR:
- PSA stands for Platform Security Architecture.
- Add support for use of psa_trusted_storage_api internal_trusted_storage.h v1.0.0
  as the interface to the psa_trusted_storage_linux backend (i.e. for persistent
  storage when MBEDTLS_PSA_ITS_FILE_C is not defined). This requires changes
  to psa_crypto_its.h and psa_crypto_storage.c to migrate to the new API.
diff --git a/library/psa_its_file.c b/library/psa_its_file.c
index 8cdf783..05ca8af 100644
--- a/library/psa_its_file.c
+++ b/library/psa_its_file.c
@@ -44,7 +44,9 @@
 #include <stdio.h>
 #include <string.h>
 
+#if !defined(PSA_ITS_STORAGE_PREFIX)
 #define PSA_ITS_STORAGE_PREFIX ""
+#endif
 
 #define PSA_ITS_STORAGE_FILENAME_PATTERN "%08lx%08lx"
 #define PSA_ITS_STORAGE_SUFFIX ".psa_its"
@@ -137,7 +139,8 @@
 psa_status_t psa_its_get( psa_storage_uid_t uid,
                           uint32_t data_offset,
                           uint32_t data_length,
-                          void *p_data )
+                          void *p_data,
+                          size_t *p_data_length )
 {
     psa_status_t status;
     FILE *stream = NULL;
@@ -172,6 +175,8 @@
     if( n != data_length )
         goto exit;
     status = PSA_SUCCESS;
+    if( p_data_length != NULL )
+        *p_data_length = n;
 
 exit:
     if( stream != NULL )