Boot floppy disk is formatted incorrectly - number of hidden sectors is wrong

As per request, this issue is being split out of CORE-5754.

When creating a ReactOS boot floppy at the end of setup, it creates the boot disk, but writes the value 3f to offset 0x1C on the boot sector of the floppy disk. This value indicates the number of hidden sectors before the FAT, and should be zero for floppy disks.

The result is that the floppy disk is unbootable. Only the error message "ldr not found" appears.

Manually correcting this value with a disk editor makes the disk bootable.

Tested in CMAKE build 53789 and this is still an issue.

HTO's comment on this from CORE-5754 was "InstallFat(16|32)BootCodeToDisk takes it from global PartitionList. That's ugly."

Note that there are other issues with the boot disk in the old Rbuild builds, but these will likely not be fixed as this build system will be going away soon.