ldlinux: CurrentDir not set if syslinux.cfg is missing

ldlinux: CurrentDir not set if syslinux.cfg is missing

If a syslinux floppy does not contain a config file (syslinux.cfg) modules are
not being found unless specified with a directory prefix (e.g. /pcitest.c32).
The reason is that CurrentDir is only set if there is a config file present. This is a
possible regression since commit 68eefb79e2cbb590ebf958dd0c50ce00b941abd2.
Before this change RootDir was used in searchdir and now it's CurrentDir unless
the filename starts with '/'. Possible fix is to set CurrentDir to RootDir if no config
file is found.

Actually it seems better to move this to the top of the "check for config file" block.
Initialize CurrentDir to RootDir and change it if a config file is found.

- Sebastian