kernel: introduce use/kernel/latest

Kernel flavours differ on secondary arches (and there's a bunch
of platform-specific kernels for ARM systems), so let's provide
some more flexible scheme than hardwiring un-def into images.

This might be further enhanced to e.g. set KFLAVOURS to
$$(KFLAVOURS_LATEST)) with it being preset to un-def
where available so rpi images could set it to rpi-un,
and still get reasonable starterkits for free.

The corresponding commit in RELENG-e2k-p9-starterkits
private branch just tore un-def apart => unmergeable.

features.in/kernel/config.mk

@@ -22,6 +22,11 @@ endif
 endif
 	@$(call xport,KFLAVOURS)
 
+use/kernel/latest: use/kernel; @:
+ifeq (,$(filter-out aarch64 armh i586 ppc64le x86_64,$(ARCH)))
+	@$(call set,KFLAVOURS,un-def)
+endif
+
 # r8168 is a kludge, never install it by default
 use/kernel/net:
 	@$(call add,THE_KMODULES,e1000e igb)