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>

Makefile

@@ -180,6 +180,12 @@ add-special-class= \
 	$(eval $(1):=) \
 	$(eval special-classes+=$(1))
 
+# Converts one or more source file paths to their corresponding build/ paths.
+# Only .c and .S get converted to .o, other files (like .ld) keep their name.
+# $1 stage name
+# $2 file path (list)
+src-to-obj=$(foreach file,$(2),$(subst .$(1),,$(basename $(patsubst src/%,$(obj)/%,$(file)))).$(1)$(patsubst %.c,%.o,$(patsubst %.S,%.o,$(suffix $(file)))))
+
 # Clean -y variables, include Makefile.inc
 # Add paths to files in X-y to X-srcs
 # Add subdirs-y to subdirs
@@ -215,12 +221,6 @@ endif
 # Eliminate duplicate mentions of source files in a class
 $(foreach class,$(classes),$(eval $(class)-srcs:=$(sort $($(class)-srcs))))
 
-# Converts one or more source file paths to their corresponding build/ paths.
-# Only .c and .S get converted to .o, other files (like .ld) keep their name.
-# $1 stage name
-# $2 file path (list)
-src-to-obj=$(foreach file,$(2),$(subst .$(1),,$(basename $(patsubst src/%,$(obj)/%,$(file)))).$(1)$(patsubst %.c,%.o,$(patsubst %.S,%.o,$(suffix $(file)))))
-
 $(foreach class,$(classes),$(eval $(class)-objs:=$(call src-to-obj,$(class),$($(class)-srcs))))
 
 # Save all objs before processing them (for dependency inclusion)