Add FILE_LICENCE declarations to almost all files that make up the
various standard builds of gPXE.

Various specification documents disagree about the byte ordering of
UUIDs.  However, SMBIOS seems to use the standard in which everything is
in network-endian order.

This doesn't affect anything sent on the wire; only what gets printed on
the screen when the "uuid" variable is displayed.

In particular, expose the system UUID as a setting ("smbios/uuid").

implemented smbios_get_uuid().