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Every addition of a new feature to the SPL header currently requires us
to update the FEL tool, to teach it about the new supported maximum
value. Many times the FEL tool doesn't really care, but complains
anyway - and refuses to load.
Let's introduce semantic versioning [1] for this field, where backwards
compatible additions just increase a minor number, but incompatible
changes require bumping the major version.
We have 8 bits for the SPL header version, let's split this to have 3 bits
for the major and 5 bit for the minor version number.

[1] https://semver.org

Signed-off-by: Andre Przywara <osp@andrep.de>
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>