s6lx9_microboard_defconfig: lock kernel version

Fixes https://travis-ci.org/buildroot/buildroot-defconfig-testing/jobs/123187960

The defconfig broke after we moved the default kernel version to 4.5.x. Fix
it by locking the kernel version in the defconfig.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

configs/s6lx9_microboard_defconfig

@@ -6,6 +6,8 @@ BR2_TARGET_GENERIC_GETTY_PORT="ttyUL0"
 # BR2_TARGET_ROOTFS_TAR is not set
 BR2_TARGET_ROOTFS_INITRAMFS=y
 BR2_LINUX_KERNEL=y
+BR2_LINUX_KERNEL_CUSTOM_VERSION=y
+BR2_LINUX_KERNEL_CUSTOM_VERSION_VALUE="4.4.7"
 BR2_LINUX_KERNEL_USE_CUSTOM_CONFIG=y
 BR2_LINUX_KERNEL_CUSTOM_CONFIG_FILE="board/avnet/s6lx9_microboard/lx9_mmu_defconfig"
 BR2_LINUX_KERNEL_USE_CUSTOM_DTS=y