linux-ext: update Xenomai help text

Reword the help text and get ride of the supported kernel
version list which is outdated since Xenomai version bump.

[Thomas: rewrap text to the appropriate length, fix some typos.]

Signed-off-by: Romain Naour <romain.naour@openwide.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>