Bump Linux kernel version to 5.15.4.
We enlarge the boot partition to follow the kernel image size increase and
update the documentation.
Signed-off-by: Vincent Stehlé <vincent.stehle@arm.com>
Reviewed-by: Dick Olsson <hi@senzilla.io>
Cc: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
