kconfig: fix savedefconfig for tristate choices

savedefconfig failed to save choice symbols equal to 'y'
for tristate choices.
This resulted in this value being lost.

In particular is fixes an issue where

	make ARCH=avr32 atngw100_defconfig
	make ARCH=avr32 savedefconfig
	cp defconfig arch/avr32/configs/atngw100_defconfig
	make ARCH=avr32 atngw100_defconfig
	diff -u .config .config.old

failed to produce an identical .config.

Signed-off-by: Sam Ravnborg <sam@ravnborg.org>
Signed-off-by: Michal Marek <mmarek@suse.cz>