test: Move fdt-overlay-test rule into test/

The Makefile rules for tests should be within test/Makefile so move the
'fdt-overlay' rule over.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 94cc3f1..d4fedd8 100644
--- a/Makefile
+++ b/Makefile
@@ -893,7 +893,6 @@
 libs-y += post/
 endif
 libs-$(CONFIG_$(PHASE_)UNIT_TEST) += test/
-libs-$(CONFIG_UT_FDT_OVERLAY) += test/fdt_overlay/
 
 libs-y += $(if $(wildcard $(srctree)/board/$(BOARDDIR)/Makefile),board/$(BOARDDIR)/)
 
diff --git a/test/Makefile b/test/Makefile
index 5ef8c37..99d4797 100644
--- a/test/Makefile
+++ b/test/Makefile
@@ -21,6 +21,7 @@
 obj-y += boot/
 obj-$(CONFIG_UNIT_TEST) += common/
 obj-$(CONFIG_UT_ENV) += env/
+obj-$(CONFIG_UT_FDT_OVERLAY) += fdt_overlay/
 obj-y += log/
 else
 obj-$(CONFIG_SPL_UT_LOAD) += image/