main/syslinux: Changed update-extlinux to detect intel-ucode

If `/boot/intel-ucode.img` is present, update-extlinux will add it as
(first) INITRD. This way the microcode update in package non-free/intel-ucode
will be automatically loaded, if installed. This package fixes, among other
processor bugs, the hyper-threading bug in Intel Skylake and Kaby Lake
processers currently in the news [1]. More details here [2] and here [3].

[1] https://lists.debian.org/debian-devel/2017/06/msg00308.html
[2] https://wiki.archlinux.org/index.php/Microcode#Syslinux
[3] https://wiki.gentoo.org/wiki/Intel_microcode