Enable flashing of kernel when kernel package is updated.

[kernel] Enable flashing of kernel when kernel package is updated.

NOTE: This is a bit hacked in as it needs device specific post scripts
as well as device specific configs however the flashing script fits all
platforms. Also should be noted that simg2img is missing and thus
flashing of all partitions is not supported.

Signed-off-by: Marko Saukko <marko.saukko@jolla.com>