[nrf noup] bootutil: set RSA key bits for PSA OAEP decrypt

PSA Core Lite does not infer key size from imported DER. Set
psa_set_key_bits() before psa_import_key() so bootutil_rsa_oaep_decrypt
accepts RSA-2048 ciphertext when validating encrypted firmware images.

Signed-off-by: Andrzej Puzdrowski <andrzej.puzdrowski@nordicsemi.no>

Author: nvlsianpu

boot/bootutil/include/bootutil/crypto/rsa.h

@@ -121,6 +121,11 @@ bootutil_rsa_parse_private_key(bootutil_rsa_context *ctx, uint8_t **p, uint8_t *
     psa_set_key_usage_flags(&key_attributes, PSA_KEY_USAGE_DECRYPT);
     psa_set_key_algorithm(&key_attributes, PSA_ALG_RSA_OAEP(PSA_ALG_SHA_256));
     psa_set_key_type(&key_attributes, PSA_KEY_TYPE_RSA_KEY_PAIR);
+#if defined(MCUBOOT_SIGN_RSA_LEN)
+    psa_set_key_bits(&key_attributes, MCUBOOT_SIGN_RSA_LEN);
+#else
+    psa_set_key_bits(&key_attributes, 2048);
+#endif
 
     status = psa_import_key(&key_attributes, *p, (end - *p), &ctx->key_id);
     return (int)status;