Implement TLS 1.3 key derivation function HKDF-Expand-Label

This commit introduces a new file library/ssl_tls13_key.c
which will subsequently be populated with functionality relating
to the TLS 1.3 key schedule.

Those functions are expected to be internal and are documented
in the internal header library/ssl_tls13_keys.h.

The first function to be implemented is the key expansion
function `HKDF-Expand-Label`. See the documentation in
library/ssl_tls13_keys.h for more information.

Signed-off-by: Hanno Becker <hanno.becker@arm.com>
diff --git a/library/Makefile b/library/Makefile
index b76a84b..bd5274d 100644
--- a/library/Makefile
+++ b/library/Makefile
@@ -162,6 +162,7 @@
 	  ssl_srv.o \
 	  ssl_ticket.o \
 	  ssl_tls.o \
+	  ssl_tls13_keys.o \
 	  # This line is intentionally left blank
 
 .SILENT: