This patch makes Limine advertise its branding and the ESP partition GUID to
systemd over the Boot Loader Interface specification. There's more to the Boot
Loader Interface than these variables, but this acts as an initial
implementation. See #473.
BLI variables are initialised for all protocols, not just Linux. While currently
only systemd makes use of it, this allows future Limine protocol kernels (or
their userspace) to discover the ESP.
