loader: abstract boot services exiting to libefi function

[imp]

Summary

Move direct call of ExitBootServices to efi_exit_boot_services. This
function sets boot_services_active to false so callers don't have to do
it everywhere (though currently only loader/bootinfo.c is affected).

Sponsored by: Netflix

Test Plan

This starts to abstract whether or not boot services are available
Though maybe we should just set BS = NULL and let callers that care use that (or better: have all callers go through a wrapper that returns an error when it's gone)