kbuild: revive single target %.ko

I removed the single target %.ko in commit ff9b45c5 ("kbuild:
modpost: read modules.order instead of $(MODVERDIR)/*.mod") because
the modpost stage does not work reliably. For instance, the module
dependency, modversion, etc. do not work if we lack symbol information
from the other modules.

Yet, some people still want to build only one module in their interest,
and it may be still useful if it is used within those limitations.

Fixes: ff9b45c5 ("kbuild: modpost: read modules.order instead of $(MODVERDIR)/*.mod")
Reported-by: Don Brace <don.brace@microsemi.com>
Reported-by: Arend Van Spriel <arend.vanspriel@broadcom.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>

Makefile

@@ -1002,6 +1002,8 @@ endif
 
 PHONY += prepare0
 
+export MODORDER := $(if $(KBUILD_EXTMOD),$(KBUILD_EXTMOD)/)modules.order
+
 ifeq ($(KBUILD_EXTMOD),)
 core-y		+= kernel/ certs/ mm/ fs/ ipc/ security/ crypto/ block/
 
@@ -1771,6 +1773,15 @@ build-dir = $(patsubst %/,%,$(dir $(build-target)))
 	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target)
 %.symtypes: prepare FORCE
 	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target)
+ifeq ($(KBUILD_EXTMOD),)
+# For the single build of an in-tree module, use a temporary file to avoid
+# the situation of modules_install installing an invalid modules.order.
+%.ko: MODORDER := .modules.tmp
+endif
+%.ko: prepare FORCE
+	$(Q)$(MAKE) $(build)=$(build-dir) $(build-target:.ko=.mod)
+	$(Q)echo $(build-target) > $(MODORDER)
+	$(Q)$(MAKE) -f $(srctree)/scripts/Makefile.modpost
 
 # Modules
 PHONY += /

scripts/Makefile.modpost

@@ -85,10 +85,8 @@ endif
 
 include scripts/Makefile.lib
 
-modorder := $(if $(KBUILD_EXTMOD),$(KBUILD_EXTMOD)/)modules.order
-
 # find all modules listed in modules.order
-modules := $(sort $(shell cat $(modorder)))
+modules := $(sort $(shell cat $(MODORDER)))
 
 # Stop after building .o files if NOFINAL is set. Makes compile tests quicker
 __modpost: $(if $(KBUILD_MODPOST_NOFINAL), $(modules:.ko:.o),$(modules))
@@ -98,7 +96,7 @@ MODPOST += $(subst -i,-n,$(filter -i,$(MAKEFLAGS))) -s -T - $(wildcard vmlinux)
 
 # We can go over command line length here, so be careful.
 quiet_cmd_modpost = MODPOST $(words $(modules)) modules
-      cmd_modpost = sed 's/ko$$/o/' $(modorder) | $(MODPOST)
+      cmd_modpost = sed 's/ko$$/o/' $(MODORDER) | $(MODPOST)
 
 PHONY += modules-modpost
 modules-modpost: