riscv: add dependency among Image(.gz), loader(.bin), and vmlinuz.efi

A common issue in Makefile is a race in parallel building.

You need to be careful to prevent multiple threads from writing to the
same file simultaneously.

Commit 3939f33450 ("ARM: 8418/1: add boot image dependencies to not
generate invalid images") addressed such a bad scenario.

A similar symptom occurs with the following command:

  $ make -j$(nproc) ARCH=riscv Image Image.gz loader loader.bin vmlinuz.efi
    [ snip ]
    SORTTAB vmlinux
    OBJCOPY arch/riscv/boot/Image
    OBJCOPY arch/riscv/boot/Image
    OBJCOPY arch/riscv/boot/Image
    OBJCOPY arch/riscv/boot/Image
    OBJCOPY arch/riscv/boot/Image
    GZIP    arch/riscv/boot/Image.gz
    AS      arch/riscv/boot/loader.o
    AS      arch/riscv/boot/loader.o
    Kernel: arch/riscv/boot/Image is ready
    PAD     arch/riscv/boot/vmlinux.bin
    GZIP    arch/riscv/boot/vmlinuz
    Kernel: arch/riscv/boot/loader is ready
    OBJCOPY arch/riscv/boot/loader.bin
    Kernel: arch/riscv/boot/loader.bin is ready
    Kernel: arch/riscv/boot/Image.gz is ready
    OBJCOPY arch/riscv/boot/vmlinuz.o
    LD      arch/riscv/boot/vmlinuz.efi.elf
    OBJCOPY arch/riscv/boot/vmlinuz.efi
    Kernel: arch/riscv/boot/vmlinuz.efi is ready

The log "OBJCOPY arch/riscv/boot/Image" is displayed 5 times.
(also "AS      arch/riscv/boot/loader.o" twice.)

It indicates that 5 threads simultaneously enter arch/riscv/boot/
and write to arch/riscv/boot/Image.

It occasionally leads to a build failure:

  $ make -j$(nproc) ARCH=riscv Image Image.gz loader loader.bin vmlinuz.efi
    [ snip ]
    SORTTAB vmlinux
    OBJCOPY arch/riscv/boot/Image
    OBJCOPY arch/riscv/boot/Image
    OBJCOPY arch/riscv/boot/Image
    OBJCOPY arch/riscv/boot/Image
    PAD     arch/riscv/boot/vmlinux.bin
  truncate: Invalid number: 'arch/riscv/boot/vmlinux.bin'
  make[2]: *** [drivers/firmware/efi/libstub/Makefile.zboot:13: arch/riscv/boot/vmlinux.bin] Error 1
  make[2]: *** Deleting file 'arch/riscv/boot/vmlinux.bin'
  make[1]: *** [arch/riscv/Makefile:167: vmlinuz.efi] Error 2
  make[1]: *** Waiting for unfinished jobs....
    Kernel: arch/riscv/boot/Image is ready
    GZIP    arch/riscv/boot/Image.gz
    AS      arch/riscv/boot/loader.o
    AS      arch/riscv/boot/loader.o
    Kernel: arch/riscv/boot/loader is ready
    OBJCOPY arch/riscv/boot/loader.bin
    Kernel: arch/riscv/boot/loader.bin is ready
    Kernel: arch/riscv/boot/Image.gz is ready
  make: *** [Makefile:234: __sub-make] Error 2

Image.gz, loader, vmlinuz.efi depend on Image. loader.bin depends
on loader. Such dependencies are not specified in arch/riscv/Makefile.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Reviewed-by: Samuel Holland <samuel.holland@sifive.com>
Tested-by: Samuel Holland <samuel.holland@sifive.com>
Link: https://lore.kernel.org/r/20231119100024.2370992-1-masahiroy@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

arch/riscv/Makefile

@@ -163,6 +163,8 @@ BOOT_TARGETS := Image Image.gz loader loader.bin xipImage vmlinuz.efi
 
 all:	$(notdir $(KBUILD_IMAGE))
 
+loader.bin: loader
+Image.gz loader vmlinuz.efi: Image
 $(BOOT_TARGETS): vmlinux
 	$(Q)$(MAKE) $(build)=$(boot) $(boot)/$@
 	@$(kecho) '  Kernel: $(boot)/$@ is ready'