karo: tx6: Finalize support for TX6Q-1020

karo: tx6: Finalize support for TX6Q-1020

- don't setup NAND related pads when CONFIG_NAND_MXS is not defined
- initialize USDHC4 interface (internal eMMC) as mmc0
- enable all LDOs
- calculate DTB partition size and number of blocks in config.mk
- enable 'fat write' command
- disable GPT and EFI partitions
- setup 'fdtsave' command to write the dtb inside the boot partition
- interpret 'splashimage' env variable as DOS filename rather than
  partition name