boards/x86: up_squared: add support for SBL(Slim Bootloader)

This adds a primitive board configuration for the UP Squared board to
work properly with SBL(Slim Bootloader).
- https://github.com/slimbootloader/slimbootloader
- https://slimbootloader.github.io/



BOARD shall be 'up_squared_sbl'
  cmake -GNinja -DBOARD=up_squared_sbl ..
  ninja

Signed-off-by: Aiden Park <aiden.park@intel.com>