rsa: Verify RSA padding programatically

Padding verification was done against static SHA/RSA pair arrays which
take up a lot of static memory, are mostly 0xff, and cannot be reused
for additional SHA/RSA pairings. The padding can be easily computed
according to PKCS#1v2.1 as:

  EM = 0x00 || 0x01 || PS || 0x00 || T

where PS is (emLen - tLen - 3) octets of 0xff and T is DER encoding
of the hash.

Store DER prefix in checksum_algo and create rsa_verify_padding
function to handle verification of a message for any SHA/RSA pairing.

Signed-off-by: Andrew Duda <aduda@meraki.com>
Signed-off-by: aduda <aduda@meraki.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

lib/sha256.c

@@ -15,6 +15,12 @@
 #include <watchdog.h>
 #include <u-boot/sha256.h>
 
+const uint8_t sha256_der_prefix[SHA256_DER_LEN] = {
+	0x30, 0x31, 0x30, 0x0d, 0x06, 0x09, 0x60, 0x86,
+	0x48, 0x01, 0x65, 0x03, 0x04, 0x02, 0x01, 0x05,
+	0x00, 0x04, 0x20
+};
+
 /*
  * 32-bit integer manipulation macros (big endian)
  */