Document CFLAGS make option

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>