riscv: Fix compressed Image formats build

make[1]: *** No rule to make target `Image.lzma'.  Stop.

When make ARCH=riscv Image.lzma, it won't work, let's fix it.
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/Makefile

@@ -96,5 +96,8 @@ $(BOOT_TARGETS): vmlinux
 	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
 	@$(kecho) '  Kernel: $(boot)/$@ is ready'
 
+Image.%: Image
+	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
+
 zinstall install:
 	$(Q)$(MAKE) $(build)=$(boot) $@