You need to escape the $< as $$<.
I think you should try to find a way to do what you want without using
more than one depth of $(call ...)
This is the order of evaluation:
$(foreach tget,$(TARGET_SYS),$(call
FileExpander,$(tget),$(TARGET_SRC_C),CVarTargetBuild))
That evaluates FileExpander:
define FileExpander
$(foreach tfile,$(2),$(eval $(call $(3),$(tfile),$(1))))
endef
With $3=CVarTargetBuild, you get:
$(foreach tfile,$(2),$(eval $(call CVarTargetBuild,$(tfile),$(1))))
The $(call CVarTargetBuild,...) here will _expand_ the variable
CVarTargetBuild. That expansion will resolve all variable references,
not just the $(1), etc. variables.
During that expansion, $< is expanded into the empty string (since that
variable only has a value when the command script is run.
So, the value you send to $(eval ...) as a result of the call doesn't
have $< in it, it just has the empty string.
Replace it with $$< and you should be OK.