kboot: Support reading the smbios vis sysfs

When reading the smbios on Linux we try to access it via /dev/mem. On
some systems we can't do this as /dev/mem is locked down. To handle
this try reading the sysfs file first, and if that fails fall back to
/dev/mem.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D51413