Provide sane Kconfig default for cmos.default.

Without that fix we have with CONFIG_USE_OPTION_TABLE:

    OPTION     cmos_layout.bin
  build/util/nvramtool/nvramtool -y /home/gnutoo/x86/coreboot-alix/src/mainboard/pcengines/alix1c/cmos.layout -L build/cmos_layout.bin
  make: *** No rule to make target `nvramtool', needed by `build/coreboot.pre1'.  Stop.
  rm build/util/sconfig/sconfig.tab.c build/cbfs/fallback/bootblock.elf build/util/sconfig/lex.yy.c

That log was captured with make V=1 but the error also appear with make.

Tested on the PC Engines ALIX.1C with the following commit (Change-Id: Ia87b090) [1]:

  PC Engines ALIX.1C: Add CMOS defaults.

[1] http://review.coreboot.org/#/c/3323/

Change-Id: I548005a58f430ed7b6da5249a24bbdcae440a1e9
Signed-off-by: Denis 'GNUtoo' Carikli <GNUtoo@no-log.org>
Reviewed-on: http://review.coreboot.org/3223
Reviewed-by: Paul Menzel <paulepanter@users.sourceforge.net>
Reviewed-by: Alexandru Gagniuc <mr.nuke.me@gmail.com>
Tested-by: build bot (Jenkins)

src/arch/x86/Kconfig

@@ -81,6 +81,7 @@ config HAVE_CMOS_DEFAULT
 
 config CMOS_DEFAULT_FILE
 	string
+	default "src/mainboard/$(MAINBOARDDIR)/cmos.default"
 	depends on HAVE_CMOS_DEFAULT
 
 config BOOTBLOCK_SOUTHBRIDGE_INIT