ARC: build: add uImage.lzma to the top-level target

arch/arc/boot/Makefile supports uImage.lzma, but you cannot do
'make uImage.lzma' because the corresponding target is missing
in arch/arc/Makefile. Add it.

I also changed the assignment operator '+=' to ':=' since this is the
only place where we expect this variable to be set.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>

arch/arc/Makefile

@@ -102,7 +102,7 @@ libs-y		+= arch/arc/lib/ $(LIBGCC)
 
 boot		:= arch/arc/boot
 
-boot_targets += uImage uImage.bin uImage.gz
+boot_targets := uImage uImage.bin uImage.gz uImage.lzma
 
 $(boot_targets): vmlinux
 	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@