CFLAGS content can be set on the command line to allow passing extra
options to the compiler. Its content is appended after the options set
by the Makefile (TF_CFLAGS).

The Makefiles must use TF_CFLAGS instead of CFLAGS, as the latter can be
completely overriden by setting it on the command line.

Also tell about LDFLAGS in the "Debugging options" section.

Change-Id: Iaf27b424002898ef3040133f78cb133983a37aee
Signed-off-by: Douglas Raillard <douglas.raillard@arm.com>