Add minimalistic HMAC_DRBG implementation

(copied from ECDSA)
diff --git a/library/Makefile b/library/Makefile
index a4126f0..311e316 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -44,6 +44,7 @@
 		ecp.o		ecp_curves.o				\
 		entropy.o	entropy_poll.o				\
 		error.o		gcm.o		havege.o		\
+		hmac_drbg.o								\
 		md.o		md_wrap.o	md2.o			\
 		md4.o		md5.o		memory.o		\
 		memory_buffer_alloc.o	net.o			\