Enforce minimum key size when generating RSA key size

Add configuration to enforce minimum size when
generating a RSA key, it's default value is 1024
bits since this the minimum secure value currently
but it can be any value greater than or equal 128
bits. Tests were modifed to accommodate for this
change.

Signed-off-by: Waleed Elmelegy <waleed.elmelegy@arm.com>

include/mbedtls/mbedtls_config.h

@@ -1335,6 +1335,15 @@
  */
 //#define MBEDTLS_RSA_NO_CRT
 
+/**
+ * \def MBEDTLS_RSA_MIN_KEY_SIZE
+ *
+ * Minimum RSA key size allowed in bits.
+ *
+ * Minimum possible value is 128 bits.
+ */
+#define MBEDTLS_RSA_MIN_KEY_SIZE 1024
+
 /**
  * \def MBEDTLS_SELF_TEST
  *