This provides a mechanism for using arbitrarily-named variables within
gPXE, using the existing syntax for settings.

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

Add SMBIOS asset tag as a named setting.

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

EFI provides a copy of the SMBIOS table accessible via the EFI system
table, which we should use instead of manually scanning through the
F000:0000 segment.