[i.MX8][D18] revert spl_board_boot_device to generic board implementation
MX8 SoCs, unlike MX6 SoCs, have different implementations of get_boot_device based on their type. Specifically, they are divided in the following groups:
- MX8MQ, MX8MM
- MX8MN, MX8MP
- MX8ULP
- other MX8
This implies different handling of the boot_device in spl_board_boot_device. Instead of moving the implementation to the specific board files, it is possible to handle the groups differences in the generic board implementation.
Edited by Gianfranco Mariotti