package/linux-headers: the latest headers are 5.8

package/linux-headers: the latest headers are 5.8

In commit c2009e9f757fe759c74406f1025967b7b8e77f0a
("package/linux-headers: license files hashes only valid for latest
version"), we introduced BR2_KERNEL_HEADERS_LATEST, which should only
be set for the most recent kernel headers versions.

Indeed, the COPYING file of Linux has changed before/after Linux 5.6,
causing its hash file to be different. Since linux-headers uses
linux/linux.hash as the hash file, and this hash file contains the
COPYING hash of Linux >= 5.6, we cannot use that hash for Linux
versions older than 5.6.

When newer versions of the headers than 5.4 were added, this
BR2_KERNEL_HEADERS_LATEST was not moved as it should have been. We fix
this, which fixes a legal-info failure happening when Linux kernel
headers 5.4 are used:

>>> linux-headers 5.4.63 Patching
>>> linux-headers 5.4.63 Collecting legal info
ERROR: COPYING has wrong sha256 hash:
ERROR: expected: fb5a425bd3b3cd6071a3a9aff9909a859e7c1158d54d32e07658398cd67eb6a0
ERROR: got     : ee5808b032a67f587d3541099d46de34f5bec8cd5976114ba07f1299ee6001ff
ERROR: Incomplete download, or man-in-the-middle (MITM) attack

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>