[rng] Add get_random_nz() function required by RSA algorithm

RSA requires the generation of random non-zero bytes (i.e. a sequence
of random numbers in the range [0x01,0xff]).  ANS X9.82 provides
various Approved methods for converting random bits into random
numbers.  The simplest such method is the Simple Discard Method.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

src/include/ipxe/random_nz.h

@@ -0,0 +1,16 @@
+#ifndef _IPXE_RANDOM_NZ_H
+#define _IPXE_RANDOM_NZ_H
+
+/** @file
+ *
+ * HMAC_DRBG algorithm
+ *
+ */
+
+FILE_LICENCE ( GPL2_OR_LATER );
+
+#include <stdint.h>
+
+extern int get_random_nz ( void *data, size_t len );
+
+#endif /* _IPXE_RANDOM_NZ_H */