lualoader: Honor boot_* variables at lua init

For non-UEFI systems, boot.config(5) may have -s or -v specified for
single-user and verbose boot respectively. These were not being properly
taken into account and reflected in the "Boot Options" submenu. When we
initialize core.lua, we'll record boot_single and boot_verbose as we do ACPI
and consider these the system defaults.

Reported by: David Wolfskill <david@catwhisker.org>
Approved by: re (kib)