Fix EFI 64bit Booting

[HarvsG]

These are all the changes I've made to get efi booting working as listed in #6.

I haven't tried the new scripts on a fresh install, so might be a good idea before merging.

Also, I haven't done the equivalent for the 32 bit solutions. It should just be a matter of replicating commits:

• [x] https://github.com/beta-tester/RPi-PXE-Server/commit/95be45373432c5714e57e0e3e1a1ba85c5fd44d9
• [x] https://github.com/beta-tester/RPi-PXE-Server/commit/c695cdf95ab559be6639e7902de0d75562ab86eb
• [x] https://github.com/beta-tester/RPi-PXE-Server/commit/be38a7341801de4d1e410763b88203b7c5faebc0
• [x] https://github.com/beta-tester/RPi-PXE-Server/commit/2e085215476b40f7fcc4c82431b0abb691af2426

[HarvsG]

Not sure what base branch you'd prefer the PR into so I've left it as master, as there were fewer conflicts.

[beta-tester]

@HarvsG, thank you for your work, testing an providing a patch... i have merged the changes to master and testing branch.