powerpc: improve prom_init_check rule

This slightly improves the prom_init_check rule.

[1] Avoid needless check

Currently, prom_init_check.sh is invoked every time you run 'make'
even if you have changed nothing in prom_init.c. With this commit,
the script is re-run only when prom_init.o is recompiled.

[2] Beautify the build log

Currently, the O= build shows the absolute path to the script:

  CALL    /abs/path/to/source/of/linux/arch/powerpc/kernel/prom_init_check.sh

With this commit, it is always a relative path to the timestamp file:

  PROMCHK arch/powerpc/kernel/prom_init_check
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20190912074037.13813-1-yamada.masahiro@socionext.com

arch/powerpc/kernel/.gitignore

+prom_init_check
 vmlinux.lds

arch/powerpc/kernel/Makefile

@@ -188,15 +188,13 @@ extra-$(CONFIG_ALTIVEC)		+= vector.o
 extra-$(CONFIG_PPC64)		+= entry_64.o
 extra-$(CONFIG_PPC_OF_BOOT_TRAMPOLINE)	+= prom_init.o
 
-ifdef CONFIG_PPC_OF_BOOT_TRAMPOLINE
-$(obj)/built-in.a:		prom_init_check
+extra-$(CONFIG_PPC_OF_BOOT_TRAMPOLINE)	+= prom_init_check
 
-quiet_cmd_prom_init_check = CALL    $<
-      cmd_prom_init_check = $(CONFIG_SHELL) $< "$(NM)" "$(obj)/prom_init.o"
+quiet_cmd_prom_init_check = PROMCHK $@
+      cmd_prom_init_check = $(CONFIG_SHELL) $< "$(NM)" $(obj)/prom_init.o; touch $@
 
-PHONY += prom_init_check
-prom_init_check: $(src)/prom_init_check.sh $(obj)/prom_init.o
-	$(call cmd,prom_init_check)
-endif
+$(obj)/prom_init_check: $(src)/prom_init_check.sh $(obj)/prom_init.o FORCE
+	$(call if_changed,prom_init_check)
+targets += prom_init_check
 
 clean-files := vmlinux.lds