Adjust dependency rules to permit per-file flags

The dependency rules are currently done in a shell 'for' loop. This does not
permit Makefile variables to adjust preprocessor flags as is done with normal
compile flags, using the CFLAGS_path/file.o syntax.

This change moves the dependency generation into the Makefile itself, and
permits a CPPFLAGS_path/file.o to adjust preprocessor flags on a file or
directory basis.

The CPPFLAGS_... variable is also folded into CFLAGS during the build.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index 44e3726..a0cafbd 100644
--- a/Makefile
+++ b/Makefile
@@ -955,7 +955,7 @@
 		| xargs rm -f
 
 clobber:	clean
-	@find $(OBJTREE) -type f \( -name '*.depend' \
+	@find $(OBJTREE) -type f \( -name '*.depend*' \
 		-o -name '*.srec' -o -name '*.bin' -o -name u-boot.img \) \
 		-print0 \
 		| xargs -0 rm -f