Sorry for the late reply but I'm very busy at work and with family.
Mario I followed your hints, anyway I'm not able to get /proc/last_kmsg
(missing file).
Before reading them I flashed twrp 2.8.6.0 and was used to reboot in recovery by long pressing power+home+volume.
Now I flashed twrp 3.0.0.0 and to get in recovery mode I simply press home + volume up while bootlooping just before the bootloader listens for
button press.
Actually /proc/last_kmsg exists but shows kmsg of the recovery itself.
Kernel config contains CONSOLE_RAM_ANDROID=y and cmdline has "console=ram", anyway I cannot set ANDROID_RAM_CONSOLE_ENABLE_VERBOSE and some
ubuntu related kernel flags (is the hlte kernel too old?).
Some suggestions?

@abdullah: Sorry but I'm quite busy to have a chat (and to maintains!), asap I'll try to write the wiki page describing the complete
instructions I followed to build UT for hlte.

I had a lot of problems too and have to hack and rehack sources, local_manifests, makefiles, deleted some device subdir (!) and so on.
If possible I'll glad to report the entire process on a wiki page of this site but I have to reorganize my notes starting a build from scratch.
Actually my full build directory is 27GB, 11GB is for the .repo and 5GB is for the out directory, so actually the only sources are 11GB.

Hi!
I was able to build (dirty) ubp 5.1 for hlte devices.
In the out/target/products/ I got the following files:
drwxr-xr-x 13 niko users 4096 11 mar 19.29 .
drwxr-xr-x 5 niko users 4096 11 mar 17.14 ..
-rw-r--r-- 1 niko users 14 10 mar 11.16 android-info.txt
-rw-r--r-- 1 niko users 49748 11 mar 19.27 clean_steps.mk
drwxr-xr-x 3 niko users 4096 10 mar 11.24 data
-rw------- 1 niko users 2195456 11 mar 19.29 dt.img
drwxr-xr-x 2 niko users 4096 10 mar 10.47 fake_packages
drwxr-xr-x 4 niko users 4096 10 mar 11.24 gen
drwxr-xr-x 3 niko users 4096 10 mar 10.37 install
-rw-r--r-- 1 niko users 53101 11 mar 19.29 installed-files.txt
-rwxr-xr-x 1 niko users 5466696 11 mar 19.29 kernel
drwxr-xr-x 20 niko users 4096 11 mar 15.22 obj
-rw-r--r-- 1 niko users 786 11 mar 19.27 previous_build_config.mk
-rw-r--r-- 1 niko users 876658 11 mar 17.21 ramdisk-android.img
-rw-r--r-- 1 niko users 4058888 11 mar 19.29 ramdisk-recovery.img
-rw-r--r-- 1 niko users 4426255 11 mar 19.27 ramdisk-ubuntu.img
drwxr-xr-x 3 niko users 4096 10 mar 10.59 recovery
-rw-r--r-- 1 niko users 11724800 11 mar 19.29 recovery.img
drwxr-xr-x 9 niko users 4096 11 mar 15.35 root
drwxr-xr-x 7 niko users 4096 10 mar 11.24 symbols
drwxr-xr-x 13 niko users 4096 10 mar 11.24 system
-rw-r--r-- 1 niko users 132591616 11 mar 19.29 system.img
drwxr-xr-x 4 niko users 4096 11 mar 19.27 ubuntu
-rw-r--r-- 1 niko users 142583344 10 mar 11.24 userdata.img
drwxr-xr-x 2 niko users 4096 10 mar 11.24 utilities

there where no boot.img, so I created it manually:
CMDLINE="console=tty selinux=0 apparmor=0"
utils/mkbootimg --kernel zImage --ramdisk ramdisk-ubuntu.img.xz --pagesize 2048 --ramdisk_offset 0x02900000 --tags_offset 0x02700000 -o boot.img --cmdline "$CMDLINE" --base 0x00000000 --dt dt.img
adb push boot.img /sdcard/
(i repacked the initrd as it's too big for the boot partition of the device, after rebuilt the kernel with XZ support)
trying to boot the device reboot after about 10 seconds, the kernel seems to be fine as if I add rootdelay=60 to cmdline the device effectively reboot one minute later and repacking boot.img with the new ubuntu kernel and the stock rom initrd boots to android.
So it seems there is a problem with the initrd as if I understand when it get problems should start adbd.
Dmesg does not show any usb activity and I was not able to get last_kmsg nor to activate the framebuffer to see kernel console messages
Suggestions? How can I investigate further?

Regards
Niko