when loopback support was introduced, it initially duplicated the code
for generating a grub2 config, before the duplicated code was removed from
the grub-pc script, effectively thus moving grub config generation to the
loopback feature script.
grub-efi support was added after this.
this results in a misleading filename, since the `binary_loopback_cfg`
script is essential for use of grub-pc|grub-efi, and actually only has a
single line of code on top that's needed for adding actual loopback
support on top. (when grub-pc and grub-efi are not used, the entire script
is still needed for loopback support to work).
so here we rename it to make better sense, and correct/clarify bits of
documentation.
Gbp-Dch: Short
a542d52141