serial-getty service: Add xterm as default TERM

In poky with systemd enabled, vt102 is selected for getty
causing user to experience a very crappy terminal. Default
TERM to xterm.

Reviewed-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Joel Fernandes <joelf@ti.com>
Signed-off-by: Saul Wold <sgw@linux.intel.com>