MIPS: zboot: Avoid useless rebuilds

Add dummy.o to the targets list, and fill targets automatically from
$(vmlinuzobjs) to avoid having to maintain two lists.

When building with XZ compression copy ashldi3.c to the build
directory to use a different object file for the kernel and zboot.
Without this the same object file need to be build with different
flags which cause a rebuild at every run.
Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Cc: Alex Smith <alex.smith@imgtec.com>
Cc: Wu Zhangjin <wuzhangjin@gmail.com>
Cc: Andrew Bresticker <abrestic@chromium.org>
Cc: linux-kernel@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/11810/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/boot/compressed/Makefile

@@ -29,8 +29,6 @@ KBUILD_AFLAGS := $(LINUXINCLUDE) $(KBUILD_AFLAGS) -D__ASSEMBLY__ \
 	-DBOOT_HEAP_SIZE=$(BOOT_HEAP_SIZE) \
 	-DKERNEL_ENTRY=$(VMLINUX_ENTRY_ADDRESS)
 
-targets := head.o decompress.o string.o dbg.o uart-16550.o uart-alchemy.o
-
 # decompressor objects (linked with vmlinuz)
 vmlinuzobjs-y := $(obj)/head.o $(obj)/decompress.o $(obj)/string.o
 
@@ -40,9 +38,13 @@ vmlinuzobjs-$(CONFIG_SYS_SUPPORTS_ZBOOT_UART16550) += $(obj)/uart-16550.o
 vmlinuzobjs-$(CONFIG_MIPS_ALCHEMY)		   += $(obj)/uart-alchemy.o
 endif
 
-ifdef CONFIG_KERNEL_XZ
-vmlinuzobjs-y += $(obj)/../../lib/ashldi3.o
-endif
+vmlinuzobjs-$(CONFIG_KERNEL_XZ) += $(obj)/ashldi3.o
+
+$(obj)/ashldi3.o: KBUILD_CFLAGS += -I$(srctree)/arch/mips/lib
+$(obj)/ashldi3.c: $(srctree)/arch/mips/lib/ashldi3.c
+	$(call cmd,shipped)
+
+targets := $(notdir $(vmlinuzobjs-y))
 
 targets += vmlinux.bin
 OBJCOPYFLAGS_vmlinux.bin := $(OBJCOPYFLAGS) -O binary -R .comment -S
@@ -60,7 +62,7 @@ targets += vmlinux.bin.z
 $(obj)/vmlinux.bin.z: $(obj)/vmlinux.bin FORCE
 	$(call if_changed,$(tool_y))
 
-targets += piggy.o
+targets += piggy.o dummy.o
 OBJCOPYFLAGS_piggy.o := --add-section=.image=$(obj)/vmlinux.bin.z \
 			--set-section-flags=.image=contents,alloc,load,readonly,data
 $(obj)/piggy.o: $(obj)/dummy.o $(obj)/vmlinux.bin.z FORCE