Fix shellcheck issue SC2154

SC2154: var is referenced but not assigned.

The 'rootmnt' variable is set in /usr/share/initramfs-tools/init and if
it's not available, that would be bad.
According to SC2154 you can fix that issue by using '${var:?}' so it
would fail if unset or empty.
So apply that and reference that SC item in the comments.

Also remove it from the exclude list in the CI config file.

debian/salsa-ci.yml

@@ -26,7 +26,7 @@ shellcheck:
   script:
     - apt-get update && apt-get upgrade -y
     - apt-get install -y shellcheck
-    - shellcheck -e SC1090,SC1091,SC2086,SC2154 -s dash $(find rootfs/etc/initramfs-tools -type f -executable  | xargs grep -l '^#!/bin/sh')
+    - shellcheck -e SC1090,SC1091,SC2086 -s dash $(find rootfs/etc/initramfs-tools -type f -executable  | xargs grep -l '^#!/bin/sh')
 
 build yamls:
   stage: build