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>
11 files changed