BUILD: Update For IAR support

Applied the same change as in mbed-crypto for using this as a sub
project with the IAR toolchain. Use __asm generic ,and avoid empty
enum. Avoid declaration of array with null size. This is a porting
of the original patch contributed to trusted-firmware-m.

Signed-off-by: TTornblom <thomas.tornblom@iar.com>
Signed-off-by: Michel Jaouen <michel.jaouen@st.com>
Signed-off-by: Antonio de Angelis <antonio.deangelis@arm.com>

CMakeLists.txt

@@ -223,7 +223,9 @@ if(CMAKE_COMPILER_IS_CLANG)
 endif(CMAKE_COMPILER_IS_CLANG)
 
 if(CMAKE_COMPILER_IS_IAR)
-    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --warn_about_c_style_casts -Ohz")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} --warn_about_c_style_casts")
+    set(CMAKE_C_FLAGS_RELEASE     "-Ohz")
+    set(CMAKE_C_FLAGS_DEBUG       "--debug -On")
 endif(CMAKE_COMPILER_IS_IAR)
 
 if(CMAKE_COMPILER_IS_MSVC)

library/psa_crypto.c

@@ -4948,8 +4948,13 @@ static psa_status_t psa_key_agreement_internal(psa_key_derivation_operation_t *o
                                                size_t peer_key_length)
 {
     psa_status_t status;
+#if PSA_KEY_AGREEMENT_MAX_SHARED_SECRET_SIZE != 0
     uint8_t shared_secret[PSA_KEY_AGREEMENT_MAX_SHARED_SECRET_SIZE];
+    size_t shared_secret_length = sizeof(shared_secret);
+#else
+    uint8_t *shared_secret = NULL;
     size_t shared_secret_length = 0;
+#endif
     psa_algorithm_t ka_alg = PSA_ALG_KEY_AGREEMENT_GET_BASE(operation->alg);
 
     /* Step 1: run the secret agreement algorithm to generate the shared
@@ -4958,7 +4963,7 @@ static psa_status_t psa_key_agreement_internal(psa_key_derivation_operation_t *o
                                             private_key,
                                             peer_key, peer_key_length,
                                             shared_secret,
-                                            sizeof(shared_secret),
+                                            shared_secret_length,
                                             &shared_secret_length);
     if (status != PSA_SUCCESS) {
         goto exit;