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>