The new functionality aims to save each file system server from having
to implement its own block I/O routines just so that it can serve as a
root file system. The new source file (bio.c) lists the requirements
that file system servers have to fulfill in order to use the routines.
Change-Id: Ia0190fd5c30e8c2097ed8f4b0e3ccde1827e0b92
