Change all doc links to use relative names

Github pages assumes that links to files using relative
names point to the same location so this should work both
when accessing through the github browser and mcuboot.com

Signed-off-by: Fabio Utzig <utzig@apache.org>
diff --git a/docs/design.md b/docs/design.md
index 24f988a..3b6e335 100644
--- a/docs/design.md
+++ b/docs/design.md
@@ -682,5 +682,4 @@
 TLV.
 
 For information on embedding public keys in the boot loader, as well as
-producing signed images, see: [signed_images]({% link signed_images.md
-%}).
+producing signed images, see: [signed_images](signed_images.md).