Fix references to trace doc

The README.trace has been moved and converted to rst in commit dce26c7d
("doc: move README.trace to HTML documentation"); fix all the remaining
references to this file.

Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Cc: Tom Rini <trini@konsulko.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>