upl: Add basic tests

Add some unit tests to check that we can write a UPL handoff and read it
back.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/test/boot/Makefile b/test/boot/Makefile
index 068522c..8ec5daa 100644
--- a/test/boot/Makefile
+++ b/test/boot/Makefile
@@ -13,3 +13,5 @@
 obj-$(CONFIG_BOOTMETH_VBE_SIMPLE) += vbe_simple.o
 endif
 obj-$(CONFIG_BOOTMETH_VBE) += vbe_fixup.o
+
+obj-$(CONFIG_UPL) += upl.o