x86: add 6.1 testing kernel

Enable building with 6.1 as testing kernel

Compile-tested: x86_64, x86_generic, x86_legacy, x86_geode
Run-tested: x86_64, x86_generic (qemu), x86_legacy (qemu)

Signed-off-by: Pascal Coudurier <[email protected]>

target/linux/x86/Makefile

@@ -11,6 +11,7 @@ FEATURES:=squashfs ext4 vdi vmdk vhdx pcmcia targz fpu boot-part rootfs-part
 SUBTARGETS:=generic legacy geode 64
 
 KERNEL_PATCHVER:=5.15
+KERNEL_TESTING_PATCHVER:=6.1
 
 KERNELNAME:=bzImage