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.

kern/include/test.h

@@ -96,6 +96,9 @@ int longstress(int, char **);
 int createstress(int, char **);
 int printfile(int, char **);
 
+/* HMAC/hash tests */
+int hmacu1(int, char**);
+
 /* other tests */
 int kmalloctest(int, char **);
 int kmallocstress(int, char **);