boot: zephyr: deprecate MBEDTLS_MD

Please use MBEDTLS_MD_C instead.
See zephyrproject-rtos/zephyr#92081

This gets rid of the
"Deprecated symbol MBEDTLS_MD is enabled."
warning when building MCUboot with Zephyr.

Signed-off-by: Stefan Gloor <stefan.gloor@siemens.com>

Author: stgloorious

boot/zephyr/Kconfig

@@ -253,7 +253,7 @@ config BOOT_SIGNATURE_TYPE_RSA
 	select BOOT_USE_MBEDTLS
 	select MBEDTLS
 	select MBEDTLS_ASN1_PARSE_C if MBEDTLS_BUILTIN
-	select MBEDTLS_MD if MBEDTLS_BUILTIN
+	select MBEDTLS_MD_C if MBEDTLS_BUILTIN
 	select MBEDTLS_RSA_C if MBEDTLS_BUILTIN
 	select MBEDTLS_PKCS1_V15 if MBEDTLS_BUILTIN
 	select MBEDTLS_PKCS1_V21 if MBEDTLS_BUILTIN