fat: calculateFatOffset derives FatFatOffset.

Assumes 512-byte sector size.
fat: calculateFatSize derives FatFatSize.

Assumes 512-byte sectors.
fat: calculateFatTotalSize derives FatFatTotalSize.
fat: calculateDataAddress derives FatDataAddress.
fat: calculateClusterSize derives FatClusterSize.
fat: calculateRootAddress derives FatRootAddress.

Assumes FatRootCluster == 2.
fat: FatDataAddress for cluster zero, not cluster one.

As described, cluster #0 and cluster #1 may not be accessed, and this
FatDataAddress actually points to an address before the data region.
fat: delete unused readSecondBlock subroutine.
fat: FatStartAddress from MBR; incorporated into other addresses.
kernal: read FAT32 root directory from SD card into RAM.
bignum.s with Uint32Multiply -> uint64 result.
fat: FatReadFile, with FatSearchFilename BSS var as param.
kernal: Display text from SPLASH.TXT on FAT32 SD card.

Tested in go5602 emulator, not on hardware.