libsa: smbios: Export the used entry point type to the kernel

Via the kenv 'smbios.entry_point_type'.

Reviewed by: imp, markj
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D49283

(cherry picked from commit 3b2303ba3dba4e33ee0866953b4390c5481cac83)