Support systemd-boot console-mode

[silkeh]

Support setting the console mode for systemd-boot. See getsolus/clr-boot-manager#5.

This is configured through /etc/kernel/console_mode. Valid values are documented in man loader.conf. When enabled, loader.conf has a console-mode line with the value, eg:

default Solus-current-6.8.12-293.conf
timeout 3
console-mode max