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/Makefile

@@ -141,6 +141,7 @@ obj-$(CONFIG_CMD_SF) += sf.o
 obj-$(CONFIG_CMD_SCSI) += scsi.o disk.o
 obj-$(CONFIG_CMD_SHA1SUM) += sha1sum.o
 obj-$(CONFIG_CMD_SETEXPR) += setexpr.o
+obj-$(CONFIG_CMD_SETEXPR_FMT) += printf.o
 obj-$(CONFIG_CMD_SPI) += spi.o
 obj-$(CONFIG_CMD_STRINGS) += strings.o
 obj-$(CONFIG_CMD_SMC) += smccc.o