os9gen: Fix bitmap check/update where boot sector is non-zero And on builds where the boot track is zero, most likely for Dragon, don't check or update the bitmap because the boot track should be reserved already. Ideally all this should be a run-time checks of the disk parameters and not depend on the build target. Later.

* Echo welcome message
echo * Welcome to NitrOS-9 Level 2 with DriveWire 4 on the Multicomp09 *
echo
* Lock shell and std utils into memory
link shell
load utilpak1
date -t