common: command: Add command execution tracer.

When using boot scripts it can become quite hard to understand
which commands are actually executed during bootup (e.g. where
is a kernel image loaded from or which DTB is in use).

Shell scripts suffer from a similar problem and many shells address
this problem with a command execution tracer (e.g. BASH has xtrace,
which can be enabled by "set -x").

This patch introduces a command tracer for U-Boot, which prints
every command with its arguments before it is executed.

Signed-off-by: Christoph Muellner <christoph.muellner@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
diff --git a/cmd/Kconfig b/cmd/Kconfig
index 2bdbfcb..f6e7cd4 100644
--- a/cmd/Kconfig
+++ b/cmd/Kconfig
@@ -53,6 +53,17 @@
 	  This string is displayed in the command line to the left of the
 	  cursor.
 
+config SYS_XTRACE
+	string "Command execution tracer"
+	depends on CMDLINE
+	default y if CMDLINE
+	help
+	  This option enables the possiblity to print all commands before
+	  executing them and after all variables are evaluated (similar
+	  to Bash's xtrace/'set -x' feature).
+	  To enable the tracer a variable "xtrace" needs to be defined in
+	  the environment.
+
 menu "Autoboot options"
 
 config AUTOBOOT