layerscape: add kernel 5.10 as testing version

This patch adds "KERNEL_TESTING_PATCHVER:=5.10" to the Makefile in
layerscape target to allow using Kernel 5.10 for testing.

Signed-off-by: Martin Schiller <ms@dev.tdt.de>

target/linux/layerscape/Makefile

@@ -7,6 +7,7 @@ include $(TOPDIR)/rules.mk
 BOARD:=layerscape
 BOARDNAME:=NXP Layerscape
 KERNEL_PATCHVER:=5.4
+KERNEL_TESTING_PATCHVER:=5.10
 FEATURES:=squashfs nand usb pcie gpio fpu ubifs ext4 rootfs-part boot-part
 SUBTARGETS:=armv8_64b armv7