discover/platform-powerpc: Handle optional Get Device ID section

The 'auxiliary' section of the 'Get Device ID' response is optional,
and some platforms exclude it from the response entirely. However
Petitboot only recognises the response as valid if it includes the full
16 bytes.
Update get_ipmi_bmc_versions() to also handle responses of only 12 bytes.

Signed-off-by: Samuel Mendoza-Jonas <sam@mendozajonas.com>