Install the syslinux UEFI bootloader.

This requires to:

1. Copy files around for compatibility between live-build 2.x and syslinux
   3:6.03~pre1+dfsg-1. Ugly, yes.
2. Install isolinux and syslinux-common explicitly, since it is the ones that
   ship isolinux.bin and the *.c32 modules in syslinux 6.x packaging, and are
   merely recommends.
Run 20-syslinux_detect_cpu with "set -e" too.

We do this for all our other hooks, and I have just seen a case when it
silently broke.
Install some COM32 syslinux modules into the syslinux directory.

Else, with syslinux 6.x, the boot fails with "ldlinux.c32 not found"
and similar.
Install an icon for rEFInd (Closes: #6529).
Merge branch 'devel' into feature/uefi
Merge remote-tracking branch 'origin/devel' into feature/uefi
Merge branch 'devel' into feature/uefi
Merge branch 'devel' into feature/uefi
Merge branch 'devel' into feature/uefi
Merge branch 'devel' into feature/uefi
Workaround syslinux menu display bugs in UEFI mode (Closes: #6576).
Merge branch 'testing' into feature/uefi
Merge branch 'devel' into feature/uefi
Merge branch 'devel' into feature/uefi
Merge branch 'devel' into feature/uefi
Merge branch 'devel' into feature/uefi
Merge branch 'devel' into feature/uefi
Adding detailed git documentation for translators
beauty operation on a sentence
small improvement, take out the alias, it's not recommended.