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/.gitignore b/.gitignore
index 0a9dc0c..320d21e 100644
--- a/.gitignore
+++ b/.gitignore
@@ -40,7 +40,7 @@
 # Generated files
 #
 
-*.depend
+*.depend*
 /LOG
 /errlog
 /reloc_off