imgtool: change getpub exporting format parameter

Update a previous PR were PEM exporting was added to the `--lang`
parameter, even though PEM is not a source code language per se.

This PR adds `--encoding/-e` to `getpub` command, for exporting
in formats other than a language source code. `--lang` is left with
a deprecation message, so it could be removed in a future version.
The default behavior of exporting source code in C was preserved.

Signed-off-by: Fabio Utzig <utzig@apache.org>