bootm: Support string substitution in bootargs

bootm: Support string substitution in bootargs

In some cases it is necessary to pass parameters to Linux so that it will
boot correctly. For example, the rootdev parameter is often used to
specify the root device. However the root device may change depending on
whence U-Boot loads the kernel. At present it is necessary to build up
the command line by adding device strings to it one by one.

It is often more convenient to provide a template for bootargs, with
U-Boot doing the substitution from other environment variables.

Add a way to substitute strings in the bootargs variable. This allows
things like "rootdev=${rootdev}" to be used in bootargs, with the
${rootdev} substitution providing the UUID of the root device.

For example, to substitute the GUID of the kernel partition:

  setenv bootargs "console=/dev/ttyS0 rootdev=${uuid}/PARTNROFF=1
		kern_guid=${uuid}"
  part uuid mmc 2:2 uuid
  bootm

This is particularly useful when the command line from another place. For
example, Chrome OS stores the command line next to the kernel itself. It
depends on the kernel version being used as well as the hardware features,
so it is extremely difficult to devise a U-Boot script that works on all
boards and kernel versions. With this feature, the command line can be
read from disk and used directly, with a few substitutions set up.

Signed-off-by: Simon Glass <sjg@chromium.org>