Fix references to trace doc

The README.trace has been moved and converted to rst in commit dce26c7d56ed
("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>
diff --git a/lib/Kconfig b/lib/Kconfig
index 37ac14f..efb7797 100644
--- a/lib/Kconfig
+++ b/lib/Kconfig
@@ -348,7 +348,7 @@
 	  Enables function tracing within U-Boot. This allows recording of call
 	  traces including timing information. The command can write data to
 	  memory for exporting for analysis (e.g. using bootchart).
-	  See doc/README.trace for full details.
+	  See doc/develop/trace.rst for full details.
 
 config TRACE_BUFFER_SIZE
 	hex "Size of trace buffer in U-Boot"