tools: ffs-test: add compatibility code for old kernels

If ffs-test is used with a kernel prior to 3.14, which do not
support the new descriptors format, it will fail when trying to
write the descriptors.  Add a function that converts the new
descriptors to the legacy ones and use it to retry writing the
descriptors using the legacy format.

Also add “-l” flag to ffs-test which will cause the tool to
never try the new format and instead immediatelly try the
legacy one.  This should be useful to test whether parsing
of the old format still works on given 3.14+ kernel.

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