samsung: board: enable support of multiple board types

This change adds declaration of functions:
- set_board_type() - called at board_early_init_f()
- get_board_type() - called at checkboard()

For supporting multiple board types in a one config - it is welcome
to display the current board model. This is what get_board_type()
should return.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Cc: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>