xt-sim: fix board definition, rename

The xt-sim defconfigs were selecting CONFIG_SIMULATOR_XTENSA instead
of CONFIG_BOARD_SIMULATOR_XTENSA.

Fix defconfigs and rename to CONFIG_BOARD_XT_SIM to ease any future
confusion between these similarly named defconfigs.

CONFIG_SIMULATOR_XTENSA is automatically set by xt-sim's
Kconfig.defconfig, it doesn't need to be explicitly specified in the
defconfigs themselves.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>