loader: improve lua ACPI detection and handling

This is a follow-up patch to https://reviews.freebsd.org/D42459
that modifies the loader lua to use the correct loader variables
for determining ACPI availability.

This also fixes a bug where ACPI can be inadvertently disabled when
setting System Defaults at the loader menu.

Reviewed by: imp, kevans
Approved by: kp
Sponsored by: Rubicon Communications, LLC ("Netgate")
Differential Revision: https://reviews.freebsd.org/D42483