ANDROID: arm64: add option to build Image-dtb

Some bootloaders couldn't decompress Image.gz-dtb.

Change-Id: I698cd0c4ee6894e8d0655d88f3ecf4826c28a645
Signed-off-by: Haojian Zhuang <haojian.zhuang@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Dmitry Shmidt <dimitrysh@google.com>
diff --git a/arch/arm64/Makefile b/arch/arm64/Makefile
index 085f792..bba8d2c 100644
--- a/arch/arm64/Makefile
+++ b/arch/arm64/Makefile
@@ -127,7 +127,7 @@
 dtbs_install:
 	$(Q)$(MAKE) $(dtbinst)=$(boot)/dts
 
-Image.gz-dtb: vmlinux scripts dtbs
+Image-dtb Image.gz-dtb: vmlinux scripts dtbs
 	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
 
 PHONY += vdso_install