Squashed commit upgrading to mbedtls-2.28.1

Squash merging branch import/mbedtls-2.28.1

 ebf1f6a58089 ("libmbedtls: compile new files added with 2.28.1")
 3ffb51b58a54 ("libmbedtls: add SM2 curve")
 c425755720b4 ("libmbedtls: mbedtls_mpi_exp_mod(): optimize mempool usage")
 23493c822a82 ("libmbedtls: mbedtls_mpi_exp_mod(): reduce stack usage")
 dcdca2348dff ("libmbedtls: mbedtls_mpi_exp_mod() initialize W")
 dc2994976958 ("libmbedtls: fix no CRT issue")
 c6628873b281 ("libmbedtls: add interfaces in mbedtls for context memory operation")
 8acd202d3e55 ("libmedtls: mpi_miller_rabin: increase count limit")
 37284e28d5d9 ("libmbedtls: add mbedtls_mpi_init_mempool()")
 b499a75f29f3 ("libmbedtls: make mbedtls_mpi_mont*() available")
 2080a8c96a5d ("mbedtls: configure mbedtls to reach for config")
 e0858334327a ("mbedtls: remove default include/mbedtls/config.h")
 dd9688e6b8ce ("Import mbedtls-2.28.1")

Signed-off-by: Jerome Forissier <jerome.forissier@linaro.org>
Acked-by: Jens Wiklander <jens.wiklander@linaro.org>
diff --git a/lib/libmbedtls/mbedtls/library/dhm.c b/lib/libmbedtls/mbedtls/library/dhm.c
index accd5a8..88e148b 100644
--- a/lib/libmbedtls/mbedtls/library/dhm.c
+++ b/lib/libmbedtls/mbedtls/library/dhm.c
@@ -231,8 +231,8 @@
         MBEDTLS_MPI_CHK( mbedtls_mpi_write_binary( ( X ),               \
                                                    p + 2,               \
                                                    ( n ) ) );           \
-        *p++ = (unsigned char)( ( n ) >> 8 );                           \
-        *p++ = (unsigned char)( ( n )      );                           \
+        *p++ = MBEDTLS_BYTE_1( n );                                     \
+        *p++ = MBEDTLS_BYTE_0( n );                                     \
         p += ( n );                                                     \
     } while( 0 )