m68k: Optimize cc-option calls for cpuflags-y

arch/m68k/Makefile computes lots of unneeded cc-option calls.

For example, if CONFIG_M5441x is not defined, there is not point in
evaluating the following compiler flag.

 cpuflags-$(CONFIG_M5441x)      := $(call cc-option,-mcpu=54455,-mcfv4e)

The result is set to cpuflags-, then thrown away.

The right hand side of ':=' is immediately expanded. Hence, all of the
16 calls for cc-option are evaluated. This is expensive since cc-option
invokes the compiler. This occurs even if you are not attempting to
build anything, like 'make ARCH=m68k help'.

Use '=' to expand the value _lazily_. The evaluation for cc-option is
delayed until $(cpuflags-y) is expanded. So, the cc-option test happens
just once at most.

This commit mimics tune-y of arch/arm/Makefile.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Acked-by: Greg Ungerer <gerg@linux-m68k.org>
Link: https://lore.kernel.org/r/20200526123810.301667-3-masahiroy@kernel.orgSigned-off-by: Geert Uytterhoeven <geert@linux-m68k.org>

arch/m68k/Makefile

@@ -32,30 +32,33 @@ endif
 #	compiler cpu type flag.
 #
 ifndef CONFIG_M68040
-cpuflags-$(CONFIG_M68060)	:= -m68060
+cpuflags-$(CONFIG_M68060)	= -m68060
 endif
 ifndef CONFIG_M68060
-cpuflags-$(CONFIG_M68040)	:= -m68040
+cpuflags-$(CONFIG_M68040)	= -m68040
 endif
-cpuflags-$(CONFIG_M68030)	:=
-cpuflags-$(CONFIG_M68020)	:=
-cpuflags-$(CONFIG_M68000)	:= -m68000
-cpuflags-$(CONFIG_M5441x)	:= $(call cc-option,-mcpu=54455,-mcfv4e)
-cpuflags-$(CONFIG_M54xx)	:= $(call cc-option,-mcpu=5475,-m5200)
-cpuflags-$(CONFIG_M5407)	:= $(call cc-option,-mcpu=5407,-m5200)
-cpuflags-$(CONFIG_M532x)	:= $(call cc-option,-mcpu=532x,-m5307)
-cpuflags-$(CONFIG_M537x)	:= $(call cc-option,-mcpu=537x,-m5307)
-cpuflags-$(CONFIG_M5307)	:= $(call cc-option,-mcpu=5307,-m5200)
-cpuflags-$(CONFIG_M528x)	:= $(call cc-option,-mcpu=528x,-m5307)
-cpuflags-$(CONFIG_M5275)	:= $(call cc-option,-mcpu=5275,-m5307)
-cpuflags-$(CONFIG_M5272)	:= $(call cc-option,-mcpu=5272,-m5307)
-cpuflags-$(CONFIG_M5271)	:= $(call cc-option,-mcpu=5271,-m5307)
-cpuflags-$(CONFIG_M523x)	:= $(call cc-option,-mcpu=523x,-m5307)
-cpuflags-$(CONFIG_M525x)	:= $(call cc-option,-mcpu=5253,-m5200)
-cpuflags-$(CONFIG_M5249)	:= $(call cc-option,-mcpu=5249,-m5200)
-cpuflags-$(CONFIG_M520x)	:= $(call cc-option,-mcpu=5208,-m5200)
-cpuflags-$(CONFIG_M5206e)	:= $(call cc-option,-mcpu=5206e,-m5200)
-cpuflags-$(CONFIG_M5206)	:= $(call cc-option,-mcpu=5206,-m5200)
+cpuflags-$(CONFIG_M68030)	=
+cpuflags-$(CONFIG_M68020)	=
+cpuflags-$(CONFIG_M68000)	= -m68000
+cpuflags-$(CONFIG_M5441x)	= $(call cc-option,-mcpu=54455,-mcfv4e)
+cpuflags-$(CONFIG_M54xx)	= $(call cc-option,-mcpu=5475,-m5200)
+cpuflags-$(CONFIG_M5407)	= $(call cc-option,-mcpu=5407,-m5200)
+cpuflags-$(CONFIG_M532x)	= $(call cc-option,-mcpu=532x,-m5307)
+cpuflags-$(CONFIG_M537x)	= $(call cc-option,-mcpu=537x,-m5307)
+cpuflags-$(CONFIG_M5307)	= $(call cc-option,-mcpu=5307,-m5200)
+cpuflags-$(CONFIG_M528x)	= $(call cc-option,-mcpu=528x,-m5307)
+cpuflags-$(CONFIG_M5275)	= $(call cc-option,-mcpu=5275,-m5307)
+cpuflags-$(CONFIG_M5272)	= $(call cc-option,-mcpu=5272,-m5307)
+cpuflags-$(CONFIG_M5271)	= $(call cc-option,-mcpu=5271,-m5307)
+cpuflags-$(CONFIG_M523x)	= $(call cc-option,-mcpu=523x,-m5307)
+cpuflags-$(CONFIG_M525x)	= $(call cc-option,-mcpu=5253,-m5200)
+cpuflags-$(CONFIG_M5249)	= $(call cc-option,-mcpu=5249,-m5200)
+cpuflags-$(CONFIG_M520x)	= $(call cc-option,-mcpu=5208,-m5200)
+cpuflags-$(CONFIG_M5206e)	= $(call cc-option,-mcpu=5206e,-m5200)
+cpuflags-$(CONFIG_M5206)	= $(call cc-option,-mcpu=5206,-m5200)
+
+# Evaluate tune cc-option calls now
+cpuflags-y := $(cpuflags-y)
 
 KBUILD_AFLAGS += $(cpuflags-y)
 KBUILD_CFLAGS += $(cpuflags-y)