raspberrypi0_defconfig: bump to kernel version 4.4.21

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

configs/raspberrypi0_defconfig

@@ -10,7 +10,7 @@ BR2_TOOLCHAIN_BUILDROOT_CXX=y
 BR2_LINUX_KERNEL=y
 BR2_LINUX_KERNEL_CUSTOM_GIT=y
 BR2_LINUX_KERNEL_CUSTOM_REPO_URL="https://github.com/raspberrypi/linux.git"
-BR2_LINUX_KERNEL_CUSTOM_REPO_VERSION="26f3b72a9c049be10e6af196252283e1f6ab9d1f"
+BR2_LINUX_KERNEL_CUSTOM_REPO_VERSION="9669a50a3a8e4f33b4fe138277bc4407e1eab9b2"
 BR2_LINUX_KERNEL_DEFCONFIG="bcmrpi"
 
 # Build the DTBs for A/B from the kernel sources: the zero is the same