[nrf noup] zephyr: Use mbedTLS specific C functions with RSA

Use snprinf, alloc, calloc and free from mbedTLS rather than
from Zephyr.

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>

Author: de-nordic

boot/zephyr/Kconfig

@@ -223,6 +223,7 @@ config BOOT_SIGNATURE_TYPE_RSA
 	select MBEDTLS
 	select MBEDTLS_ASN1_PARSE_C if MBEDTLS_BUILTIN
 	select MBEDTLS_KEY_EXCHANGE_RSA_ENABLED if MBEDTLS_BUILTIN
+	select MBEDTLS_PLATFORM_NO_STD_FUNCTIONS if MBEDTLS_BUILTIN
 	select BOOT_ENCRYPTION_SUPPORT
 	select BOOT_IMG_HASH_ALG_SHA256_ALLOW
 	select BOOT_AES_MBEDTLS_DEPENDENCIES if MBEDTLS_BUILTIN && BOOT_ENCRYPT_IMAGE