cmd: env: Add `indirect` to indirectly set values

This allows an ergonomic-enough approximation of ${!variable} expansion.
This could be used the following way:

```
for target in ${boot_targets}; do
   env indirect target_name target_name_${target}
   # ...
done
```

Assuming `target_name_mmc0` and similar are set appropriately.

A default value can be optionally provided.

Note: this acts on environment variables, not hush variables.

Signed-off-by: Samuel Dionne-Riel <samuel@dionne-riel.com>
Cc: Simon Glass <sjg@chromium.org>
Cc: "Marek Behún" <marek.behun@nic.cz>
[trini: Don't enable by default]