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>
diff --git a/scripts/config.py b/scripts/config.py
index bb4a22c..ef13062 100755
--- a/scripts/config.py
+++ b/scripts/config.py
@@ -110,6 +110,8 @@
     'MBEDTLS_TEST_CONSTANT_FLOW_MEMSAN', # build dependency (clang+memsan)
     'MBEDTLS_TEST_CONSTANT_FLOW_VALGRIND', # build dependency (valgrind headers)
     'MBEDTLS_X509_REMOVE_INFO', # removes a feature
+    'MBEDTLS_PSA_STATIC_KEY_SLOTS', # only relevant for embedded devices
+    'MBEDTLS_PSA_STATIC_KEY_SLOT_BUFFER_SIZE', # only relevant for embedded devices
 ])
 
 def is_seamless_alt(name):