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/cmd/Kconfig b/cmd/Kconfig
index 3830519..f9f2716 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -2832,8 +2832,8 @@
 	  Enables a command to control using of 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.
+	  for analysis (e.g. using bootchart). See doc/develop/trace.rst
+	  for full details.
 
 config CMD_AVB
 	bool "avb - Android Verified Boot 2.0 operations"