image: Add signing infrastructure

Add a structure to describe an algorithm which can sign and (later) verify
images.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/common/Makefile b/common/Makefile
index 6aa41a4..48791b7 100644
--- a/common/Makefile
+++ b/common/Makefile
@@ -243,6 +243,7 @@
 COBJS-y += image.o
 COBJS-$(CONFIG_OF_LIBFDT) += image-fdt.o
 COBJS-$(CONFIG_FIT) += image-fit.o
+COBJS-$(CONFIG_FIT_SIGNATURE) += image-sig.o
 COBJS-y += memsize.o
 COBJS-y += stdio.o