UEFI

You can replace BOOT.EFI with anything that is properly formed for armv7a (and self signed). See Compiling grub2 You can boot into a UEFI shell using shell.efi from here https://github.com/tianocore/edk2/tree/UDK2018/ShellBinPkg/UefiShell/Arm

RT2 lsefisystab (efi system tables)

Notes

https://forum.xda-developers.com/showpost.php?p=82555449&postcount=126 coherixmatts - "This has been helpful to me, as I was able to quickly test changes to the BCD file. Changing the BCD file allowed me to replace test.efi with bootshim.efi from here: github.com/imbushuo/boot-shim/releases

Instead of loading EFI files like test.efi, this alternative loads ELF files. That isn't necessarily so exciting, but this BootShim also unloads elements of the original SecureBoot restricted UEFI. They are using this on Lumia devices (particularly the 950XL) to load a modified version of UEFI. After that, it seems they can boot most anything." This thread also seems interesting for reference https://github.com/rhboot/shim/issues/138