[nrf fromlist] zephyr: rsa: Use a smaller sha256 implementation

Reduce the size of the sha256 implementation by 1300 bytes by using a
slower implementation.

mcu-tools/mcuboot#1283

Signed-off-by: Sebastian Bøe <sebastian.boe@nordicsemi.no>
(cherry picked from commit 296af28)

Author: SebastianBoe

boot/zephyr/include/config-rsa.h

@@ -64,6 +64,7 @@
 #define MBEDTLS_MD_C
 #define MBEDTLS_OID_C
 #define MBEDTLS_SHA256_C
+#define MBEDTLS_SHA256_SMALLER
 #define MBEDTLS_SHA224_C
 #define MBEDTLS_AES_C