syslinux: bump to version 4.06

Signed-off-by: Daniel Price <daniel.price@gmail.com>
Tested-by: Will Moore <will.moore@beraninstruments.com>
Tested-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>