explain BDS and the custom `CONFIG_EFI_BDS` patch etc

[orangecms]

The website and also https://github.com/linuxboot/linuxboot are referring to a CONFIG_EFI_BDS option that was once based on a custom patch; see also: https://github.com/linuxboot/linuxboot/issues/96

OTOH, https://github.com/linuxboot/book/blob/main/src/implementation.md explains steps to implement LinuxBoot on top of a vendor provided UEFI PI, which also involves BDS, but pictures a different approach again. People looking for details would get lost here. We need to elaborate on the whole thing.