[smbios] Allow access to unreferenced SMBIOS strings

iPXE allows access to general SMBIOS settings using the syntax:

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

This provides access to any fixed-offset field within an SMBIOS
structure.  This syntax is currently overloaded to interpret a zero
<length> as meaning that the byte at <offset> contains a string index;
this provides access to SMBIOS strings (which are not located at fixed
offsets).

The "OEM Strings" SMBIOS structure contains strings which are not
referenced by any fixed string index field within the structure.  iPXE
currently provides no way to access these strings.

Fix by overloading the syntax for numerical SMBIOS settings to
interpret an <offset> of zero as implying that <length> contains a
literal string index.  The OEM Strings can then be accessed using:

  smbios/11.0.1
  smbios/11.0.2
  smbios/11.0.3
  ...

The actual byte at offset zero will always contain the structure type,
which is already known since it must be specified in order to access
the structure.  There is thus no plausible existing use case for an
offset of zero; overloading the syntax in this way should therefore
not break compatibility with any existing scripts.

The corner case where both <offset> and <length> are zero is undefined
(and, for now, will simply return a "not found" error).

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