configs/README.txt: Improve example with MBEDTLS/TF_PSA_CRYPTO_CONFIG_FILE

Signed-off-by: Ronald Cron <[email protected]>

Author: ronald-cron-arm

configs/README.txt

@@ -15,12 +15,14 @@ To use one of these pairs, you can pick one of the following methods:
 1. Replace the default files include/mbedtls/mbedtls_config.h and
    tf-psa-crypto/include/psa/crypto_config.h with the chosen ones.
 
-2. Use the MBEDTLS_CONFIG_FILE and TF_PSA_CRYPTO_CONFIG_FILE options of the
-   CMake build system:
+2. Use the MBEDTLS_CONFIG_FILE and TF_PSA_CRYPTO_CONFIG_FILE CMake options. For
+   example, to build out-of-tree with the config-ccm-psk-tls1_2.h and
+   crypto-config-ccm-psk-tls1_2.h configuration pair:
 
-   cmake -DMBEDTLS_CONFIG_FILE="path-to-your-mbedtls-config-file" \
-         -DTF_PSA_CRYPTO_CONFIG_FILE="path-to-your-tf-psa-crypto-config-file" .
-   make
+   cmake -DMBEDTLS_CONFIG_FILE="configs/config-ccm-psk-tls1_2.h" \
+         -DTF_PSA_CRYPTO_CONFIG_FILE="configs/crypto-config-ccm-psk-tls1_2.h"
+         -B build-psktls12 .
+   cmake --build build-psktls12
 
 The second method also works if you want to keep your custom configuration
 files outside the Mbed TLS tree.