cmd: setexpr: add format string handling

Add format string handling operator to the setexpr command.
It allows to use C or Bash like format string expressions to be
evaluated with the result being stored inside the environment variable
name.

  setexpr <name> fmt <format> [value]...

The following example

  setexpr foo fmt "%d, 0x%x" 0x100 ff

will result in $foo being set to "256, 0xff".

Signed-off-by: Roland Gaudig <roland.gaudig@weidmueller.com>
Reviewed-by: Simon Glass <sjg@chromium.org>

cmd/printf.h

@@ -0,0 +1,8 @@
+/* SPDX-License-Identifier: GPL-2.0+ */
+
+#ifndef __PRINTF_H
+#define __PRINTF_H
+
+int printf_setexpr(char *str, size_t size, int argc, char *const *argv);
+
+#endif