bootctl: load /etc/kernel/install.conf's $layout field, too

bootctl: load /etc/kernel/install.conf's $layout field, too

bootctl so far tried to determine the layout from /etc/machine-info, but
that's obsolete, and the kernel-install script never looked there. Let's
read the setting from /etc/kernel/install.conf instead, where
kernel-install actually looks.

Support for reading the field from /etc/machine-info is retained for
compat.

This means we'll now read /etc/machine-id, /etc/machine-info and
/etc/kernel/install.conf, and read the machine ID from the former too
and the layout setting from the latter two.