um: xterm: Make default terminal emulator configurable

Make the default terminal emulator configurable so e.g.
Debian can set it to x-terminal-emulator instead of the
current default of xterm.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Acked-By: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Tested-by: Ritesh Raj Sarraf <ritesh@collabora.com>
Signed-off-by: Richard Weinberger <richard@nod.at>