upl: Add support for writing a upl handoff

Universal Payload provides a standard way of handing off control between
two firmware phases. Add support for writing the handoff information from
a structure.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/boot/Makefile b/boot/Makefile
index 9e4e5a6..f4675d6 100644
--- a/boot/Makefile
+++ b/boot/Makefile
@@ -45,6 +45,7 @@
 
 obj-$(CONFIG_$(SPL_TPL_)UPL) += upl_common.o
 obj-$(CONFIG_$(SPL_TPL_)UPL_READ) += upl_read.o
+obj-$(CONFIG_$(SPL_TPL_)UPL_WRITE) += upl_write.o
 
 obj-$(CONFIG_$(SPL_TPL_)OF_LIBFDT) += image-fdt.o
 obj-$(CONFIG_$(SPL_TPL_)FIT_SIGNATURE) += fdt_region.o