`/etc/kernel/cmdline` can be used to store additional kernel
command-line parameters that might be required for a specific target.
However, this file is ignored by `u-boot-update`, which only sources
`/etc/default/u-boot`.

This would require different processes for u-boot targets and other
ones. Therefore, having both kinds of targets using the contents of
`/etc/kernel/cmdline` would improve portability.

This commit imports the `cmdline` file and appends its contents to the
parameters found in `/etc/default/u-boot`.

Signed-off-by: Arnaud Ferraris <arnaud.ferraris@collabora.com>