Simply select a kernel for your data disk to convert it to a bootable system disk. You'll still be able to attach and mount a system disk to a server like a data disk.

In your disk's management page, click the “Associate to a kernel” link:

In the Kernel Selection section, open the dropdown menu to select a kernel version.

You can leave the default command line options untouched or adapt them to fit your needs.

To boot a server using one of its disks, make sure you convert it to a bootable disk first using the instructions above.

Then, from the “Disk Management” section in your server's admin page, click the radio button presented in the “Boot” column. An alert window will pop up to prompt you for confirmation.