[source] build: fix another regression in append-dtb fix

[LEDE Commits]

nbd pushed a commit to source.git, branch master:
https://git.lede-project.org/c6f71ad30b768266a8fcc42af719881e93739365

commit c6f71ad30b768266a8fcc42af719881e93739365
Author: Felix Fietkau <nbd at nbd.name>
AuthorDate: Fri Nov 3 16:08:12 2017 +0100

    build: fix another regression in append-dtb fix
    
    Filter out duplicate generated code for the same dts files
    
    Signed-off-by: Felix Fietkau <nbd at nbd.name>
---
 include/image.mk | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/include/image.mk b/include/image.mk
index 2d7b1ef..96a8892 100644
--- a/include/image.mk
+++ b/include/image.mk
@@ -437,10 +437,14 @@ endef
 
 ifndef IB
 define Device/Build/dtb
+  ifndef BUILD_DTS_$(1)
+  BUILD_DTS_$(1) := 1
   $(KDIR)/image-$(1).dtb: FORCE
 	$(call Image/BuildDTB,$(strip $(2))/$(1).dts,$$@)
 
   image_prepare: $(KDIR)/image-$(1).dtb
+  endif
+
 endef
 endif