make: support "make ssl-opt" to just build what ssl-opt.sh needs

This also suffices for compat.sh.

Include the sample programs in this build. They aren't tested by ssl-opt.sh
yet, but they soon will be.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/Makefile b/Makefile
index fb80529..00de608 100644
--- a/Makefile
+++ b/Makefile
@@ -28,6 +28,9 @@
 programs: lib mbedtls_test
 	$(MAKE) -C programs
 
+ssl-opt: lib mbedtls_test
+	$(MAKE) -C programs ssl-opt
+
 lib:
 	$(MAKE) -C library