Rename run_command() to builtin_run_command()

The current run_command() is only one of the parsing options - the other
is hush. We should not call run_command() when the hush parser is being
used. So we rename this function to better explain its purpose.

Signed-off-by: Simon Glass <sjg@chromium.org>
Acked-by: Mike Frysinger <vapier@gentoo.org>