tools: ffs-test: convert to new descriptor format fixing compilation error

tools: ffs-test: convert to new descriptor format fixing compilation error

Commit [1ec2b885: “usb: gadget: f_fs: Add flags to descriptors block”]
which introduced a new descriptor format for FunctionFS removed the
usb_functionfs_descs_head structure, which is still used by ffs-test.
tool.

Convert ffs-test by converting it to use the new header format.  For
testing kernels prior to 3.14 (when the new format was introduced) and
parsing of the legacy headers in the new kernels, provide a compilation
flag to make the tool use the old format.

Finally, include information as to when the legacy FunctionFS headers
format has been deprecated (which is also when the new one has been
introduced).

Reported-by: Lad, Prabhakar <prabhakar.csengg@gmail.com>
Signed-off-by: Michal Nazarewicz <mina86@mina86.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>