efi: switch boot_services_gone to boot_services_active

Turn the presence or absence of boot services into a positive bool (and
change its type to bool). Move declaration to efi.h in the global
variables section.

Sponsored by: Netflix

Reviewed by: tsoome, kib
Differential Revision: https://reviews.freebsd.org/D31814

(cherry picked from commit 305ef653bcf349c7dea83c90add6f2d97910e545)