MD: isolate strings to separate table

In the long term, we don't really want those strings in the library.
Start with isolating them in a separate table rather than having them in
the main md_info structure.

This way, the table can easily be garbage-collected by the linker if
none of the two functions using it are called.

Also, simplify the implementation of mbedtls_md_info_from_string().

This saves 151 bytes with MD_C, and 141 with MD_LIGHT only.

Signed-off-by: Manuel Pégourié-Gonnard <manuel.pegourie-gonnard@arm.com>

library/md_wrap.h

@@ -39,9 +39,6 @@ extern "C" {
  * Allows message digest functions to be called in a generic way.
  */
 struct mbedtls_md_info_t {
-    /** Name of the message digest */
-    const char *name;
-
     /** Digest identifier */
     mbedtls_md_type_t type;