HP UEFI doesn't boot Ubuntu automatically

I own a HP ProBook, which came with SLED11 preinstalled, and on which I has Trusty installed
(NO Dual boot). UEFI wasn't enabled. Now I bought a SSD and built it into the laptop. As I read that using UEFI can improve firmware loading time by several seconds, I enabled it and booted the Ubuntu Trusty Live USB I created with UNetBootin.

After the installation finished (erase disk and install), I rebooted and noticed that the HP boot menu said it didn't find any OS's. When I open up the Boot device selection, it lists the following:

OS Boot Manager
Boot from EFI file
ubuntu

The entry ubuntu boots my system. I can also manually specify the path to grubx64.efi. But OS Boot Manager shows above error message.

How do I get my Ubuntu automatically booted, without hitting F9 every time?

PS: The UEFI setup allows me to specify a path to a custom EFI file, if that might be helpful. However, /EFI/ubuntu/grubx64.efi doesn't work. I assume it requires some sort of device ID before it... But I's prefer putting Ubuntu in the OS Boot Manager.

Edit 1: It turns out that this is a flaw in HP's UEFI implementation. It seems that Customized Boot is the only option.

Edit 2: As soon as the HP guys tell me how to apply a firmware upgrade to my computer (refuses to load the update EFI image), I'll do so, maybe OS Boot Manager will boot Ubuntu then.

Edit 3: I learned it is not possible. To upgrade the firmware of my ProBook 455, I need to run Windows directly on it, as the upgrade image is generated locally by a Windows-only software, which reads various stuff from the BIOS, so I can neither use virtualization nor Wine.

The path that needs to be entered as the Custom Boot image is:

When using Secure Boot: EFI\ubuntu\shimx64.efi
When not using Secure Boot: EFI\ubuntu\grubx64.efi

This is one of two workarounds. The other would be to copy /boot/efi/EFI/ubuntu/(grub|shim)x64.efi to /boot/efi/EFI/BOOT/bootx64.efi.

Source: https://bugs.launchpad.net/linuxmint/+bug/1309395