The name of the bootloader is syslinux, while isolinux is just one
of many components. isolinux.bin now also accepts syslinux.cfg
as a configuration file name (as do all other loaders).

Thus, rename the isolinux/ folder to syslinux/, and rename
isolinux.cfg to syslinux.cfg. The only occurrence of 'isolinux'
is now the actual loader file 'isolinux.bin'. This makes
the transition from isolinux to the other syslinux loaders
easier when remastering the ISO onto another medium.