I've just bought a Dell XPS 17 (l702x) and I'm interested in multi-booting a variety of OSes. If what I've understood is correct, the Dell has some form of locked-down Phoenix SecureCore Tiano UEFI 'BIOS'. From what I've read, UEFI isn't directly usable (possibly via a hidden menu etc., which might require a BIOS mod).
It does seem possible to use/access a Phoenix compatible EFI shell, using the open-source (BSD) TianoCore edk2/ShellPkg (source) and edk2/ShellBinPkg (binary) packages (GIT Repo here). Lending from a TianoCore/UEFI post I found:
(TianoCore/EDK) ShellBinPkg offers a "full" UEFI Shell 2.0 (which supports the most commands). A custom shell can be built using the ShellPkg source code (standalone or included in an OVMF package to generate a x64 version).
The [U]EFI shell binary is compiled to run independent of the firmware. This can be tested by putting the shell on a FAT32 file system (USB stick, hard drive partition), renamed as "/efi/boot/bootx64.efi" and then booting to it, from your [UEFI] BIOS.
Help text for the shell is accessed by typing "help utilname". Just using "help" produces a list of all available shell commands.
Also of interest is Arch Linux's take on UEFI, which mentions the following, supporting the use of a shell, as above:
Note: If you are unable to launch UEFI Shell from the firmware directly, create a FAT32 USB pen drive with Shell.efi copied as (USB)/efi/boot/bootx64.efi . This USB should come up in the firmware boot menu. Launching this option will launch the UEFI Shell for you.