Move the declaration of the functions needed to use the test
implementation of mbedtls_psa_external_get_random() to a new header
file. Before, they were declared in
tests/include/test/psa_crypto_helpers.h, but this header file can't be
included in sample programs because it also includes headers from the
library directory which is not on the include path for sample
programs.
This fixes the build of the sample programs when
MBEDTLS_PSA_CRYPTO_EXTERNAL_RNG and MBEDTLS_USE_PSA_CRYPTO are
enabled.
Move the implementation of the functions to a separate .c file as
well. This isn't strictly necessary, but makes the structure of the
source code easier to understand.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
