board:samsung: check the boot device and init the right mmc driver.

board:samsung: check the boot device and init the right mmc driver.

It is possible to boot device using a micro SD or eMMC slots.
In this situation, boot device should be registered as a block
device 0 in the MMC framework, because CONFIG_SYS_MMC_ENV_DEV
is usually set to "0" in the most config cases.

Signed-off-by: Przemyslaw Marczak <p.marczak@samsung.com>
Change-Id: I39c654005138824c8acdfe4458f5f45294c902fe
---
Changes V3:
- separate two changes into two commits

Changes V4:
- board.c: add functions: init_mmc() and init_dwmmc()
- board_mmc_init(): call get_boot_mode()

Changes V5:
- none

Changes V6:
- update boot mode name