kboot: Support reading the smbios vis sysfs

[andrew]

Summary

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.

Sponsored by: Arm Ltd

[imp]

Yea. Seems fine.