tools: ffs-test: convert to new descriptor format

Since commit [ac8dde11: “Add flags to descriptors block”] functionfs
supports a new, more powerful and extensible, descriptor format.
Since ffs-test is probably the first thing users of the functionfs
interface see when they start writing functionfs user space daemons,
convert it to use the new format thus promoting it.

Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>