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Use TF_LDFLAGS from the Makefiles, and still append LDFLAGS as well to
the compiler's invocation. This allows passing extra options from the
make command line using LDFLAGS.

Document new LDFLAGS Makefile option.

Change-Id: I88c5ac26ca12ac2b2d60a6f150ae027639991f27
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>