kernal: loadSplashScreen loads into $6000 for displayText.

Previously it had its own displayText-like implementation.
kernal: deleted display of raw SD card data; splash screen only.
fat: WIP; read FAT32 params from SD card into kernal_bss vars.

Testing with:

go6502 -via-ssd1306 -sd-card=sd4gb.fat32 \
  -debug -debug-symbol-file=build/debug \
  -debug-commands="
    break-address Halt;
    continue;
    read16 FatSectorSize;
    read FatSectorsPerCluster;
    read16 FatReservedSectors;
    read FatFatCount;
    read16 FatSectorsPerFat;
    read16 FatRootCluster;
    quit
    "