Added sha256 hash function and hmac functions (with and without salt) to the kernel

and userspace.

In userspace, this is accessed through #include <secure.h>
In the kernel, this is access through #include <kern/secure.h>

There is a unit test for this (hm1) that computes the hmac and compares it to
the known value.  The salted vesion tested offline.

-----

Also, fixed usespace compile issue with not changing KERNEL_SECRET => SECRET.

common/libc/secure/secure.h

@@ -0,0 +1,16 @@
+#ifndef _SECURE_H
+#define _SECURE_H
+
+#define SHA256_BLOCK_SIZE 64
+#define SHA256_OUTPUT_SIZE 32
+
+#define TOHEX(n) n < 10 ? '0'+n : 'a' + (n-10)
+
+// Compute the hex string from SHA256 hash output
+void hex_from_hash(unsigned char hash[SHA256_OUTPUT_SIZE], char res[SHA256_OUTPUT_SIZE*2 + 1]);
+
+// Compute the FIPS 198-1 complient HMAC of msg using SHA256
+void hmac_sha256(const char *msg, size_t msg_len, char *key, size_t key_len, 
+		unsigned char output[SHA256_OUTPUT_SIZE]);
+
+#endif //_SECURE_H