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.

userland/lib/libc/Makefile

@@ -13,6 +13,11 @@ SRCS+=\
 	$(COMMON)/printf/snprintf.c \
 	$(COMMON)/printf/tprintf.c
 
+# secure
+SRCS+=\
+	$(COMMON)/secure/secure.c \
+	$(COMMON)/secure/sha256.c \
+
 # stdio
 SRCS+=\
 	stdio/__puts.c \