[boot] Create distribution /bootopts file larger than 512 bytes

[ghaerr]

Extends the distribution /bootopts file to larger than 512 bytes (max size still 1023 bytes). This was done for two reasons:

• The /bootopts loader for FAT always reads two sectors, but if only one is present, a garbage sector (the next consecutive sector) will also be read. If the /bootopts file is exactly 512 bytes long, then it won't be zero terminated and the garbage sector will be interpreted at startup.
• If the /bootopts file is edited on a FAT filesystem, the new /bootopts will only work if the file was already two sectors in size, thus guaranteeing the file sectors remain contiguous. When editing /bootopts on FAT, the file size should not be reduced below 513 bytes, as doing so may cause the file contents to become non-contiguous when next edited.

Comments were added to the xtide= line for informational purposes:

#xtide=3 # 0=ATA, 1=XTIDEv1, 2=XTIDEv2 (high speed), 3=XTCF