Generalize dh_flag in mbedtls_mpi_gen_prime

Setting the dh_flag to 1 used to indicate that the caller requests safe
primes from mbedtls_mpi_gen_prime. We generalize the functionality to
make room for more flags in that parameter.
diff --git a/tests/suites/test_suite_mpi.data b/tests/suites/test_suite_mpi.data
index 227c472..5ff3f93 100644
--- a/tests/suites/test_suite_mpi.data
+++ b/tests/suites/test_suite_mpi.data
@@ -706,7 +706,7 @@
 
 Test mbedtls_mpi_gen_prime (Safe)
 depends_on:MBEDTLS_GENPRIME
-mbedtls_mpi_gen_prime:128:1:0
+mbedtls_mpi_gen_prime:128:MBEDTLS_MPI_GEN_PRIME_FLAG_DH:0
 
 Test bit getting (Value bit 25)
 mbedtls_mpi_get_bit:10:"49979687":25:1