kbuild: fix building bzImage with CONFIG_TRIM_UNUSED_KSYMS enabled

When building a specific target such as bzImage, modules aren't normally
built.  However if CONFIG_TRIM_UNUSED_KSYMS is enabled, no built modules
means none of the exported symbols are used and therefore they will all
be trimmed away from the final kernel.  A subsequent "make modules" will
fail because modpost cannot find the needed symbols for those modules in
the kernel binary.

Let's make sure modules are also built whenever CONFIG_TRIM_UNUSED_KSYMS
is enabled and that the kernel binary is properly rebuilt accordingly.
Signed-off-by: Nicolas Pitre <nico@linaro.org>
Tested-by: Jarod Wilson <jarod@redhat.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

Makefile

@@ -607,6 +607,13 @@ else
 include/config/auto.conf: ;
 endif # $(dot-config)
 
+# For the kernel to actually contain only the needed exported symbols,
+# we have to build modules as well to determine what those symbols are.
+# (this can be evaluated only once include/config/auto.conf has been included)
+ifdef CONFIG_TRIM_UNUSED_KSYMS
+  KBUILD_MODULES := 1
+endif
+
 # The all: target is the default when no target is given on the
 # command line.
 # This allow a user to issue only 'make' to build a kernel including modules
@@ -944,7 +951,7 @@ ifdef CONFIG_GDB_SCRIPTS
 endif
 ifdef CONFIG_TRIM_UNUSED_KSYMS
 	$(Q)$(CONFIG_SHELL) $(srctree)/scripts/adjust_autoksyms.sh \
-	  "$(MAKE) KBUILD_MODULES=1 -f $(srctree)/Makefile vmlinux_prereq"
+	  "$(MAKE) -f $(srctree)/Makefile vmlinux"
 endif
 
 # standalone target for easier testing