Add setlocal, update PATH with LIGHTFIELD_ROOT

[FreddieAkeroyd]

• Add setlocal to stop PATH getting too long on multiple runs of start_epics.bat
• Add LIGHTFIELD_ROOT to PATH to stop potential DLL issues, though it doesn't fix the current issue

I am assuming that adding LIGHTFIELD_ROOT will avoid rather than cause potential problems, but i don't have experience with the device

[ericonr]

@FreddieAkeroyd are you still interested in merging this? They look like small changes