lib/process: Add add_stderr flag to process module

For some process execution functions, we'd like to capture stderr as
well as stdout. Currently, we unconditionally redirect subprocess stderr
to the petitboot log file.

This change adds an add_stderr flag to struct process, which indicates
to the process library that we want stderr as well as stdout. If this is
specified, the subprocess' stderr is captured to stdout_buf.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>