RFE: add support for multiple second stage loaders

If a distribution wants to support multiple second stage bootloaders, it is hard with the current hardcoded filename (either grub<arch>.efi or the override of DEFAULT_LOADER set at compilation time). Please add support for having multiple second-stage bootloaders installed. The goal is to decouple shim from the second stage bootloader, so that the user can install one or more of the bootloader packages and the new efi binary will be picked up automatically, but at the same time, the bootloader packages don't step on each other's toes and can be updated / installed / removed independently.

I think this could be implemented as a glob or a prefix pattern so it's possible to add new second-stage bootloaders without modifying shim itself. E.g. Fedora could set DEFAULT_LOADERS=fedora-*.efi, and then shim would list all efi binaries present in ESP, and for the ones that match the glob, try them in turn. Or if a glob is too hard to implement, DEFAULT_LOADER_PREFIX=fedora-, and then try to boot anything that starts with the prefix and ends with .efi.

Start by tracing how the hardcoded grub.efi name and compile-time DEFAULT_LOADER are used to select a second-stage bootloader. Then investigate EFI binaries present on the ESP and define how matching loaders should be tried independently; done means multiple installed bootloader packages can be installed, updated, and removed without stepping on each other.