LinuxKPI: Implement efi_enabled(EFI_BOOT)

efi_enabled(EFI_BOOT) returns true if machine was booted from EFI
Used by drm-kmod and some unported drm drivers like hyperv

MFC after: 1 week
Reviewed by: hselasky, manu
Differential Revision: https://reviews.freebsd.org/D33300