psa: allow to use static key buffers instead of dynamic ones

This helps reducing heap memory usage and, if heap memory is
not used anywhere else in an embedded device, it also reduces
code footprint since there is no need for heap management code
in this case.

A new build symbol is added for this purpose, named
MBEDTLS_PSA_STATIC_KEY_SLOTS. It's disabled by default so that
normal usage of Mbed TLS library is not affected.

Signed-off-by: Valerio Setti <valerio.setti@nordicsemi.no>

include/mbedtls/mbedtls_config.h

@@ -3066,6 +3066,39 @@
  */
 #define MBEDTLS_PSA_ITS_FILE_C
 
+/**
+ * \def MBEDTLS_PSA_STATIC_KEY_SLOTS
+ *
+ * Statically preallocate all key slot buffers to store volatile keys in PSA
+ * instead of allocating them dynamically when required. This helps reducing
+ * heap memory usage as well as heap management code's footprint in embedded
+ * devices.
+ *
+ * \note This feature comes with a (potentially) higher RAM usage since:
+ *       - All the key slots are allocated no matter if they are used of not.
+ *       - Each key slot's length is as large as the largest key type supported
+ *         in the build.
+ *
+ * Requires: MBEDTLS_PSA_CRYPTO_C
+ *
+ */
+//#define MBEDTLS_PSA_STATIC_KEY_SLOTS
+
+/**
+ * \def MBEDTLS_PSA_STATIC_KEY_SLOTS
+ *
+ * Optionally define the size (in bytes) of each static key slot. If not
+ * explicitly defined then it's automatically set to hold the maximum
+ * asymmetric PSA key enabled in the build (through PSA_WANT_xxx symbols).
+ * If required by the application this parameter can be set to higher values
+ * in order to store larger objects (ex: raw keys), but please note that this
+ * will increase RAM usage.
+ *
+ * Requires: MBEDTLS_PSA_STATIC_KEY_SLOTS
+ *
+ */
+//#define MBEDTLS_PSA_STATIC_KEY_SLOT_BUFFER_SIZE       256
+
 /**
  * \def MBEDTLS_RIPEMD160_C
  *