Deal with --force-extra-removable with signed shim too

In this case, we need both the signed shim as /EFI/BOOT/BOOTXXX.EFI
and signed Grub as /EFI/BOOT/grubXXX.efi.

Also install the BOOTXXX.CSV into /EFI/debian, and FBXXX.EFI into
/EFI/BOOT/ so that it can work when needed (*iff* we're updating the
NVRAM).

[cjwatson: Refactored also_install_removable somewhat for brevity and so
that we're using consistent case-insensitive logic.]

Bug-Debian: https://bugs.debian.org/930531
Last-Update: 2021-09-24

Patch-Name: grub-install-removable-shim.patch