Support booting from MBR disks

Adds (optional) MBR logic that sets the base sector and sector count used by the redoxfs module dynamically, from the four MBR partitions, rather than including the file system directly and using labels. This is backwards-compatible, since it's only enabled when neither FILESYSTEM nor KERNEL is set, in which case it previously simply wouldn't have worked.

mentioned in merge request redox!1227 (merged)

merged

mentioned in commit 97b1ae39