Add EXT filesystem support to SPL

Hi,

I have prepared a patch to add EXT filesystem support to SPL, but I have some questions to make a good patch.

Currently, we have : MMCSD_MODE_FAT for FAT boot. Should we add MMCSD_MODE_EXT or just rename it to MMCSD_MODE_FS (or something else) to be filesystem generic?

Then, some vars are defined using FAT in their name such as: CONFIG_SYS_MMC_SD_FAT_BOOT_PARTITION or CONFIG_SPL_FAT_LOAD_PAYLOAD_NAME. We could use them also for EXT but FAT in the name is a bit annoying. Defining new vars seems useless to me, so I propose to just remove FAT from the name.

Once I get feedback on this, I will send a proper patch.

Regards,

Guillaume