-obj-y := stdio_console.o fd.o chan_kern.o chan_user.o line.o
+ifndef CONFIG_UMMODE_LIB
+obj-y := stdio_console.o
+else
+obj-y :=
+endif
+obj-y += fd.o chan_kern.o chan_user.o line.o

Might nicer to do via Kconfig, such as

config STDIO_CONSOLE
	def_bool y
	depends on !UMMODE_LIB

and then

obj-$(CONFIG_STDIO_CONSOLE) += stdio_console.o

here. Similar to CONFIG_STDDER_CONSOLE, after all.

+/**
+ * lkl_print - optional operation that receives console messages

How is it optional? I don't see you having a __weak definition?

johannes