kbuild: add verbose option to Section mismatch reporting in modpost

If the config option CONFIG_SECTION_MISMATCH is not set and
we see a Section mismatch present the following to the user:

modpost: Found 1 section mismatch(es).
To see additional details select "Enable full Section mismatch analysis"
in the Kernel Hacking menu (CONFIG_SECTION_MISMATCH).

If the option CONFIG_SECTION_MISMATCH is selected
then be verbose in the Section mismatch reporting from mdopost.
Sample outputs:

WARNING: o-x86_64/vmlinux.o(.text+0x7396): Section mismatch in reference from the function discover_ebda() to the variable .init.data:ebda_addr
The function  discover_ebda() references
the variable __initdata ebda_addr.
This is often because discover_ebda lacks a __initdata
annotation or the annotation of ebda_addr is wrong.

WARNING: o-x86_64/vmlinux.o(.data+0x74d58): Section mismatch in reference from the variable pci_serial_quirks to the function .devexit.text:pci_plx9050_exit()
The variable pci_serial_quirks references
the function __devexit pci_plx9050_exit()
If the reference is valid then annotate the
variable with __exit* (see linux/init.h) or name the variable:
*driver, *_template, *_timer, *_sht, *_ops, *_probe, *_probe_one, *_console,

WARNING: o-x86_64/vmlinux.o(__ksymtab+0x630): Section mismatch in reference from the variable __ksymtab_arch_register_cpu to the function .cpuinit.text:arch_register_cpu()
The symbol arch_register_cpu is exported and annotated __cpuinit
Fix this by removing the __cpuinit annotation of arch_register_cpu or drop the export.
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>

lib/Kconfig.debug

@@ -108,6 +108,8 @@ config DEBUG_SECTION_MISMATCH
 	    will tell where the mismatch happens much closer to the
 	    source. The drawback is that we will report the same
 	    mismatch at least twice.
+	  - Enable verbose reporting from modpost to help solving
+	    the section mismatches reported.
 
 config DEBUG_KERNEL
 	bool "Kernel debugging"

scripts/Makefile.modpost

@@ -62,6 +62,7 @@ modpost = scripts/mod/modpost                    \
  $(if $(KBUILD_EXTMOD),-i,-o) $(kernelsymfile)   \
  $(if $(KBUILD_EXTMOD),-I $(modulesymfile))      \
  $(if $(KBUILD_EXTMOD),-o $(modulesymfile))      \
+ $(if $(CONFIG_DEBUG_SECTION_MISMATCH),,-S)      \
  $(if $(KBUILD_EXTMOD)$(KBUILD_MODPOST_WARN),-w)
 
 quiet_cmd_modpost = MODPOST $(words $(filter-out vmlinux FORCE, $^)) modules