cmd: xtrace: Convert to bool

This variable is a boolean, not a string.

Signed-off-by: Sean Anderson <sean.anderson@seco.com>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index c735e81..f4cff0c 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -65,7 +65,7 @@
 	  to complete a command. Usually "> ".
 
 config SYS_XTRACE
-	string "Command execution tracer"
+	bool "Command execution tracer"
 	depends on CMDLINE
 	default y if CMDLINE
 	help