linux: use backtick instead of $(shell ...) make function

Verified that LINUX_VERSION_PROBED is only used in "-quoted commands
(actually, usually it's not quoted).

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>