nitrogen6x: fixup linux tarball setting

A terminating '"' was missing.

Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

configs/nitrogen6x_defconfig

@@ -21,6 +21,6 @@ BR2_TARGET_UBOOT_CUSTOM_TARBALL_LOCATION="http://github.com/boundarydevices/u-bo
 # kernel
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_TARBALL=y
-BR2_LINUX_KERNEL_CUSTOM_TARBALL_LOCATION="http://github.com/boundarydevices/linux-imx6/tarball/5809938/linux-imx6-5809938.tar.gz
+BR2_LINUX_KERNEL_CUSTOM_TARBALL_LOCATION="http://github.com/boundarydevices/linux-imx6/tarball/5809938/linux-imx6-5809938.tar.gz"
 BR2_LINUX_KERNEL_DEFCONFIG="nitrogen6x"
 BR2_LINUX_KERNEL_INSTALL_TARGET=y