linux: fix fbtft kernel extension

linux: fix fbtft kernel extension

Fixes fbtft kernel extension bug reported by Richard Fergusson ([1]):

drivers/video/Kconfig:2525: can't open file
"drivers/video/fbdev/fbtft/Kconfig"

Fix: write the right fbtft/KConfig path to video/Kconfig or
video/fbdev/Kconfig (instead of hard coded one)

[1] http://lists.busybox.net/pipermail/buildroot/2015-January/117057.html

Reported-by: Richard Fergusson <fergie4000@gmail.com>
Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>