I was able to update without problems, rebooted into 10.12.6 with the fakeid for skylake. Then edit the config.plist to switch to native kaby lake (following advice here), rebooted again, it didn't boot. Now even if I put the old (skylake injecting) config.plist I get:
Mach-O file has bad magic number 0x0
Error Loading kernel cache (0x1)
Boot failed, sleeping for 10 seconds before exiting..
I'm able to mount the hfs+ partition as read-only from Linux and it seems that it is fine.
I've tried, -f -x -s with the same result, any idea?
Edit: boot with my 10.12.5 install USB and chroot + kextcache -update-volume seems to fix it, at least for the old skylake config.plist
Edit2: it seems that the first time it booted but without graphics, so when I hardreset I interrupted the kextcache auto update corrupting the precompiled kernel image. Changing the ig-platform-id to 0x59160000 to and the IntelGFX FakeID to 0x59168086 fixed it. Alienware 13 R3 working as MacBookPro 14,3