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/main/menu.c

@@ -491,6 +491,7 @@ static const char *testmenu[] = {
 	"[fs4] FS write stress 2             ",
 	"[fs5] FS long stress                ",
 	"[fs6] FS create stress              ",
+	"[hm1] HMAC unit test                ",
 	NULL
 };
 
@@ -649,6 +650,9 @@ static struct {
 	{ "fs4",	writestress2 },
 	{ "fs5",	longstress },
 	{ "fs6",	createstress },
+
+	/* HMAC unit tests */
+	{ "hm1",	hmacu1 },
 	
 #if OPT_AUTOMATIONTEST
 	/* automation tests */