configs/raspberrypi0_defconfig: bump version to 4.9.21

Signed-off-by: Martin Bark <martin@barkynet.com>
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="935c7ce84c982a26f567a03a58a1537424569938"
+BR2_LINUX_KERNEL_CUSTOM_REPO_VERSION="5e4ee836560d4c0371e109bf469e1ad808ae7a44"
 BR2_LINUX_KERNEL_DEFCONFIG="bcmrpi"
 
 # Build the DTBs for A/B from the kernel sources: the zero is the same