kbuild: define KBUILD_MODNAME even if multiple modules share objects

Currently, KBUILD_MODNAME is defined only when $(modname) contains
just one word.  If an object is shared among multiple modules,
undefined KBUILD_MODNAME could cause a build error.  For example,
if CONFIG_DYNAMIC_DEBUG is enabled, any call of printk() populates
.modname, then fails to build due to undefined KBUILD_MODNAME.

Take the following code as an example:

  obj-m += foo.o
  obj-m += bar.o
  foo-objs := foo-bar-common.o foo-only.o
  bar-objs := foo-bar-common.o bar-only.o

In this case, there is room for argument what to define for
KBUILD_MODNAME when foo-bar-common.o is being compiled.
"foo", "bar", or what else?

One idea is to define colon-separated modules that share the object,
in this case, "bar:foo" (modules are sorted alphabetically by
$(sort ...)).

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Cao jin <caoj.fnst@cn.fujitsu.com>

scripts/Makefile.lib

@@ -87,13 +87,9 @@ subdir-ym	:= $(addprefix $(obj)/,$(subdir-ym))
 # These flags are needed for modversions and compiling, so we define them here
 # $(modname_flags) defines KBUILD_MODNAME as the name of the module it will
 # end up in (or would, if it gets compiled in)
-# Note: Files that end up in two or more modules are compiled without the
-#       KBUILD_MODNAME definition. The reason is that any made-up name would
-#       differ in different configs.
 name-fix = $(squote)$(quote)$(subst $(comma),_,$(subst -,_,$1))$(quote)$(squote)
 basename_flags = -DKBUILD_BASENAME=$(call name-fix,$(basetarget))
-modname_flags  = $(if $(filter 1,$(words $(modname))),\
-                 -DKBUILD_MODNAME=$(call name-fix,$(modname)))
+modname_flags  = -DKBUILD_MODNAME=$(call name-fix,$(modname))
 
 orig_c_flags   = $(KBUILD_CPPFLAGS) $(KBUILD_CFLAGS) \
                  $(ccflags-y) $(CFLAGS_$(basetarget).o)
@@ -174,8 +170,10 @@ dtc_cpp_flags  = -Wp,-MD,$(depfile).pre.tmp -nostdinc                    \
 		 -undef -D__DTS__
 
 # Finds the multi-part object the current object will be linked into
-modname-multi = $(sort $(foreach m,$(multi-used),\
-		$(if $(filter $*.o, $($(m:.o=-objs)) $($(m:.o=-y))),$(m:.o=))))
+# If the object belongs to two or more multi-part objects, all of them are
+# concatenated with a colon separator.
+modname-multi = $(subst $(space),:,$(sort $(foreach m,$(multi-used),\
+		$(if $(filter $*.o, $($(m:.o=-objs)) $($(m:.o=-y))),$(m:.o=)))))
 
 # Useful for describing the dependency of composite objects
 # Usage: