boot.asm: retry on read error

[boeckmann]

Currently, the FAT12/16 bootloader does not try to re-read on error. However, this should be done, as floppy drives tend to produce occasional read errors. For example, on my system, boot fails sometimes because of this.

[boeckmann]

Should be solved by 71fc092. But I had to remove the "dots" output, as I was short of a single byte. If anyone finds a byte to spare in boot.asm, I can re-enable the output.