init: introduce ro.kernel.version property

This property will hold the major.minor part of the kernel version (e.g. "5.4"), allowing init scripts to act depending on that version, enabling and disabling certain features.

Bug: 194156700
Change-Id: Icec640b8a7150b344d9aa3bc0bdbcdae050c7c45
Test: manual on a Pixel device
Signed-off-by: Alexander Potapenko <glider@google.com>