lib/rsa: Make fdt_add_bignum() available outside of RSA code

fdt_add_bignum() is useful for algorithms other than just RSA. To
allow its use for ECDSA, move it to a common file under lib/.

The new file is suffixed with '-libcrypto' because it has a direct
dependency on openssl. This is due to the use of the "BIGNUM *" type.

Signed-off-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

include/u-boot/fdt-libcrypto.h

@@ -0,0 +1,27 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+/*
+ * Copyright (c) 2020, Alexandru Gagniuc <mr.nuke.me@gmail.com>
+ * Copyright (c) 2013, Google Inc.
+ */
+
+#ifndef _FDT_LIBCRYPTO_H
+#define _FDT_LIBCRYPTO_H
+
+#include <openssl/bn.h>
+
+/**
+ * fdt_add_bignum() - Write a libcrypto BIGNUM as an FDT property
+ *
+ * Convert a libcrypto BIGNUM * into a big endian array of integers.
+ *
+ * @blob:	FDT blob to modify
+ * @noffset:	Offset of the FDT node
+ * @prop_name:	What to call the property in the FDT
+ * @num:	pointer to a libcrypto big number
+ * @num_bits:	How big is 'num' in bits?
+ * @return 0 if all good all working, -ve on horror
+ */
+int fdt_add_bignum(void *blob, int noffset, const char *prop_name,
+		   BIGNUM *num, int num_bits);
+
+#endif /* _FDT_LIBCRYPTO_H */