mmc: fsl_esdhc_spl: pre-PBL: check for BOOT signature instead of MBR/DBR
Pre-PBL BootROMs (MPC8536E, MPC8569E, P2020, P1011, P1012, P1013, P1020,
P1021, P1022) require custom BOOT signature on sector 0 and MBR/DBR
signature is not required at all.
So add check for BOOT signature and remove check for MBR/DBR.
This allows U-Boot SPL to load proper U-Boot on pre-PBL BootROMs platforms
also from SD cards which do not have MBR/DBR signature on sector 0.