imx: imx6ul: kontron-sl-mx6ul: Select correct boot and env device

Instead of checking both, SPI NOR and MMC for loading U-Boot proper
and the environment, implement a way to detect the actual boot
device even if the BootROM doesn't report it and we can't rely
solely on the fuse settings, as by default we use MMC as primary
boot device and boot from SPI NOR via the secondary fallback device
(EEPROM Recovery Mode).

Signed-off-by: Frieder Schrempf <frieder.schrempf@kontron.de>