Move object files to $(obj)/<class>/

Instead of tagging object files with .<class>, move them to a <class>
directory below $(obj)/. This way we can keep a 1:1 mapping between
source- and object-file names.

The 1:1 mapping is a prerequisite for Ada, where the compiler refuses
any other object-file name.

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

Change-Id: Idb7a8abec4ea0a37021d9fc24cc8583c4d3bf67c
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/13181
Reviewed-by: Aaron Durbin <adurbin@chromium.org>
Tested-by: build bot (Jenkins)

Makefile

@@ -184,7 +184,13 @@ add-special-class= \
 # 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)))))
+src-to-obj=\
+	$(patsubst $(obj)/%,$(obj)/$(1)/%,\
+	$(patsubst $(obj)/$(1)/%,$(obj)/%,\
+	$(patsubst src/%,$(obj)/%,\
+	$(patsubst %.c,%.o,\
+	$(patsubst %.S,%.o,\
+	$(subst .$(1),,$(2)))))))
 
 # Clean -y variables, include Makefile.inc
 # Add paths to files in X-y to X-srcs