net: wan: wanxl: use allow to pass CROSS_COMPILE_M68k for rebuilding firmware

As far as I understood from the Kconfig help text, this build rule is
used to rebuild the driver firmware, which runs on an old m68k-based
chip. So, you need m68k tools for the firmware rebuild.

wanxl.c is a PCI driver, but CONFIG_M68K does not select CONFIG_HAVE_PCI.
So, you cannot enable CONFIG_WANXL_BUILD_FIRMWARE for ARCH=m68k. In other
words, ifeq ($(ARCH),m68k) is false here.

I am keeping the dead code for now, but rebuilding the firmware requires
'as68k' and 'ld68k', which I do not have in hand.

Instead, the kernel.org m68k GCC [1] successfully built it.

Allowing a user to pass in CROSS_COMPILE_M68K= is handier.

[1] https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/9.2.0/x86_64-gcc-9.2.0-nolibc-m68k-linux.tar.xz

Suggested-by: Geert Uytterhoeven <geert@linux-m68k.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

drivers/net/wan/Kconfig

@@ -200,7 +200,7 @@ config WANXL_BUILD_FIRMWARE
 	depends on WANXL && !PREVENT_FIRMWARE_BUILD
 	help
 	  Allows you to rebuild firmware run by the QUICC processor.
-	  It requires as68k, ld68k and hexdump programs.
+	  It requires m68k toolchains and hexdump programs.
 
 	  You should never need this option, say N.
 

drivers/net/wan/Makefile

@@ -40,17 +40,17 @@ $(obj)/wanxl.o:	$(obj)/wanxlfw.inc
 
 ifeq ($(CONFIG_WANXL_BUILD_FIRMWARE),y)
 ifeq ($(ARCH),m68k)
-  AS68K = $(AS)
-  LD68K = $(LD)
+  M68KAS = $(AS)
+  M68KLD = $(LD)
 else
-  AS68K = as68k
-  LD68K = ld68k
+  M68KAS = $(CROSS_COMPILE_M68K)as
+  M68KLD = $(CROSS_COMPILE_M68K)ld
 endif
 
 quiet_cmd_build_wanxlfw = BLD FW  $@
       cmd_build_wanxlfw = \
-	$(CPP) -D__ASSEMBLY__ -Wp,-MD,$(depfile) -I$(srctree)/include/uapi $< | $(AS68K) -m68360 -o $(obj)/wanxlfw.o; \
-	$(LD68K) --oformat binary -Ttext 0x1000 $(obj)/wanxlfw.o -o $(obj)/wanxlfw.bin; \
+	$(CPP) -D__ASSEMBLY__ -Wp,-MD,$(depfile) -I$(srctree)/include/uapi $< | $(M68KAS) -m68360 -o $(obj)/wanxlfw.o; \
+	$(M68KLD) --oformat binary -Ttext 0x1000 $(obj)/wanxlfw.o -o $(obj)/wanxlfw.bin; \
 	hexdump -ve '"\n" 16/1 "0x%02X,"' $(obj)/wanxlfw.bin | sed 's/0x  ,//g;1s/^/static const u8 firmware[]={/;$$s/,$$/\n};\n/' >$(obj)/wanxlfw.inc; \
 	rm -f $(obj)/wanxlfw.bin $(obj)/wanxlfw.o