Ship syslinux.cfg instead of isolinux.cfg in the ISO

Reported on IRC:

(18:13:25) Ady2: anonym: I've been trying to bring attention to a boot issue in EFI in TAILS.
(18:13:34) Ady2: In the ISO images, TAILS is using "/EFI/BOOT/isolinux.cfg". This is incorrect (there is no "isolinux" in UEFI). It should be "EFI/BOOT/syslinux.cfg".
(18:13:55) Ady2: For example, to solve the problem described in https://mailman.boum.org/pipermail/tails-testers/2014-October/000075.html the user needs to copy "/EFI/BOOT/isolinux.cfg" to "/EFI/BOOT/syslinux.cfg".
(18:14:04) Ady2: There are more potential improvements (or better ones), but copying "/EFI/BOOT/isolinux.cfg" to "/EFI/BOOT/syslinux.cfg" is the less stressful / minimal interference one.

Related issues

Is duplicate of #12439 (closed)

Original created by @anonym on 8144 (Redmine)

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