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Currently, .d files are generated before any objects are built.
So, IS_ANYTHING_TO_BUILD flag is needed to avoid such processing for
non-build targets.

There is a cleverer way; just create a .d file simultaneously when
the corresponding object is created.  No need to have separate rules
for .d files.

This commit will also fix a bug; -D$(IMAGE) is defined for $(OBJ),
but not for $(PREREQUISITES).  So, .d files are generated with
different macro sets from those for .o files, then wrong .d files
are generated.

For example, in lib/cpus/aarch64/cpu_helpers.S

  #if IMAGE_BL31
  #include <cpu_data.h>
  #endif

<cpu_data.h> is parsed for the object when built for BL31, but the
.d file does not pick up that dependency.

With this commit, the compiler will generate .o and .d at the same
time, guaranteeing they are generated under the same circumstances.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>