Makefile: Make full use of src-to-obj macro

There were several spots in the tree where the path to a per class
object file was hardcoded. To make use of the src-to-obj macro for
this, it had to be moved before the inclusion of subdirs. Which is
fine, as it doesn't have dependencies beside $(obj).

Tested by verifying that the resulting coreboot.rom files didn't change
for all of Jenkins' abuild configurations.

Change-Id: I2eb1beeb8ae55872edfd95f750d7d5a1cee474c4
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/13180
Tested-by: build bot (Jenkins)
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

src/cpu/ti/am335x/Makefile.inc

@@ -22,7 +22,7 @@ $(eval $(call create_class_compiler,omap-header,arm))
 
 real-target: $(obj)/MLO
 
-header_ld = $(obj)/cpu/ti/am335x/header.omap-header.ld
+header_ld := $(call src-to-obj,omap-header,$(dir)/header.ld)
 
 get_header_size= \
 	$(eval omap_header_info=$(shell $(CBFSTOOL) $(1) print | grep $(2))) \