buildroot

qmx6/wandboard_defconfig: switch to custom kernel headers version

Since 3.0.x was EOL and deprecated this would cause it to use newer
headers than the kernel and cause issues.
So switch to custom kernel headers version string to fix this.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>