docs: add /loader/entries.srel to the boot loader spec

docs: add /loader/entries.srel to the boot loader spec

This new file is supposed to address conflicts with Fedora/Grub's
frankenbootloaderspec implementation, that squatted the /loader/entries/
dir, but place incompatible files in them (that do variable expansion?).

A simple text file /loader/entries.srel shall indicate which spec is
implemented. If it contains the string "type1\n" then the
/loader/entries/ directory implements our standard spec, otherwise
something else.