package/linux-backports: bump version to 5.8

Attempting to compile this package with newer Kernel version (e.g. v5.4)
fails with message:

   Generating local configuration database from kernel ...Kernel version parse failed!

Upgrading the package to 5.8 fixes this issue. Anyways, v4.4 is now
rather old and beat the very purpose of having newer drivers in older
kernels.

Since backports tag v4.14-rc4-1, the requirement on minimal kernel
version changed from 3.0 to 3.10. See commit [1]. The minimal kernel
version check is changed accordingly.

License files are also updated: the linux backports package copies the
license files from the kernel version used for its generation. v5.8 is
now "GPL-2.0 WITH Linux-syscall-note". However, there is no such SPDX
identifier (contrary to what is said in the COPYING file), so we keep it
as GPL-2.0 (which also keeps it aligned to what we have in linux.mk).

[1] https://git.kernel.org/pub/scm/linux/kernel/git/backports/backports.git/commit/?id=a0d05f9f9ca50ea8b1d60726fac6b54167257e76



Signed-off-by: Julien Olivain <ju.o@free.fr>
Reviewed-by: Petr Vorel <petr.vorel@gmail.com>
Tested-by: Petr Vorel <petr.vorel@gmail.com>
[yann.morin.1998@free.fr: keep license as GPL-2.0, like for linux]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>