lkl: avoid using file function of make for older GNU make

With make version 4.0, make -f Makefile.autoconf crashes, and with
version 3.8, it will not create the output file (Makefile.conf) at all.

This issues are due to the (relatively) new feature of GNU make, the
file function.

This commit fixes those issue by rewriting file function statements by
echo commands.

This also fixes torvalds#403.

Fixes: 2030bf6 ("lkl tools: improve the build system")

Signed-off-by: Hajime Tazaki <thehajime@gmail.com>

tools/lkl/Makefile.autoconf

@@ -2,8 +2,9 @@ POSIX_HOSTS=elf64-x86-64 elf32-i386 elf64-x86-64-freebsd elf32-littlearm elf64-l
 NT_HOSTS=pe-i386 pe-x86-64
 
 define set_autoconf_var
-  $(file >> $(OUTPUT)/include/autoconf.h,#define LKL_HOST_CONFIG_$(1) $(2))
-  $(file >> $(OUTPUT)/tests/autoconf.sh,LKL_HOST_CONFIG_$(1)=$(2))
+  $(shell echo "#define LKL_HOST_CONFIG_$(1) $(2)" \
+	  >> $(OUTPUT)/include/autoconf.h)
+  $(shell echo "LKL_HOST_CONFIG_$(1)=$(2)" >> $(OUTPUT)/tests/autoconf.sh)
   export LKL_HOST_CONFIG_$(1)=$(2)
 endef
 
@@ -85,7 +86,10 @@ define do_autoconf
   $(if $(filter $(LD_FMT),$(NT_HOSTS)),$(call nt_host,$(LD_FMT)))
 endef
 
+export do_autoconf
+
+
 $(OUTPUT)Makefile.conf: Makefile.autoconf
-	$(file > $(OUTPUT)/include/autoconf.h)
-	$(file > $(OUTPUT)/tests/autoconf.sh)
-	$(file > $(OUTPUT)/Makefile.conf,$(call do_autoconf))
+	$(shell echo -n "" > $(OUTPUT)/include/autoconf.h)
+	$(shell echo -n "" > $(OUTPUT)/tests/autoconf.sh)
+	@echo "$$do_autoconf" > $(OUTPUT)/Makefile.conf