Add a pair of functions which can be used to allocate and free
an HMAC_CTX structure on the heap. This means that the caller doesn't
need to know the size of the underlying structure.
All source files in lib/hcrypto should be built the same way.
Since this source directory is dependent on libroken then all source
files must be built using the roken.h declarations and included headers.
Also, there is no config.h in the local directory so angle brackets
include of quotes should be used.
Finally, because roken.h includes stdio.h, stdlib.h, stdarg.h, limits.h,
strings.h, sys/types.h, etc., do not include them separately.
Start all source files with
#include <config.h>
#include <roken.h>
Change-Id: I09ab47f8a5472018efe6c8b59a0e51fde8f24724
