[smbios] Allow access to multiple instances of SMBIOS structures

[smbios] Allow access to multiple instances of SMBIOS structures

Extend the syntax for numerical SMBIOS settings from

  smbios/<type>.<offset>.<length>

to

  smbios/[<instance>.]<type>.<offset>.<length>

Where SMBIOS provides multiple structures with the same <type>, this
extended syntax allows for access to structures other than the first.
If <instance> is omitted then it will default to zero, giving access
to the first instance (and so matching existing behaviour).

The 16-bit SMBIOS handle (which is an alternative way to disambiguate
multiple instances of the same type of structure) can be accessed, if
required, using

  smbios/<instance>.<type>.2.2:uint16

Signed-off-by: Michael Brown <mcb30@ipxe.org>