riscv: Clean up boot dir

Let's remove all files under riscv boot dir by using archclean rule.
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/Makefile

@@ -101,3 +101,6 @@ Image.%: Image
 
 zinstall install:
 	$(Q)$(MAKE) $(build)=$(boot) $@
+
+archclean:
+	$(Q)$(MAKE) $(clean)=$(boot)

arch/riscv/boot/Makefile

@@ -18,7 +18,7 @@ KCOV_INSTRUMENT := n
 
 OBJCOPYFLAGS_Image :=-O binary -R .note -R .note.gnu.build-id -R .comment -S
 
-targets := Image loader
+targets := Image Image.* loader loader.o loader.lds loader.bin
 
 $(obj)/Image: vmlinux FORCE
 	$(call if_changed,objcopy)