x86: use sysfs DMI information to populate sysinfo

Use the DMI data available in sysfs to extract manufacturer and model info
and write it to /tmp/sysinfo/.

The data will be picked up by board_detect and can be used by e.g. LuCI to
display a more appropriate model description.

On an APU board the files will contain the following values:

    # cat /tmp/sysinfo/model
    PC Engines APU

    # cat /tmp/sysinfo/board_name
    pc-engines-apu

Signed-off-by: Jo-Philipp Wich <jo@mein.io>