I tried everything here and nothing worked, but I did find a solution and this solution does NOT require access to another Mac! It's the same method from Mavericks, but it will only work for iCloud/App Store (iMessage doesn't work):
1. Go to system preferences, network, click on the Ethernet network adapter, click the minus button at the bottom, hit apply.*
2. Go to /Library/Preferences/System Configuration/NetworkInterfaces.Plist and delete it or move it somewhere else. (just in case)
3. Open up EFI studio (Download: http://cl.ly/2s3Y0K1n0d2g2s222V1P), change device to Ethernet, then click add device. Click write to com.apple.boot.plist then put the app to the side for now.*
4. Copy this:
5. Paste that in your org.Chameleon.boot.plist file which is located in /extra/org.chameleon.boot.plist. The key and the string match up with the others. Go back to the EFI Studio editor.
6. Copy the long number at the bottom and paste it in between the two strings in your chameleon boot plist file.
7. Save & Restart*
8. Go in to system preferences, then network, then click the plus button at the bottom. 9. Click add in the window that pops up. It should say "Ethernet". If it has "en1" or "en0" after it, then repeat the above steps.
10. After adding that hit apply and restart one last time.*
11. Go to Safari to check that you're getting internet connectivity, then continue signing into iCloud and/or the App Store!