fix(kernel-install): do not generate an initrd when one was specified

According to the synopsis, kernel-install can be called with an
already-prepared initrd. In that case, no initrd should be generated by dracut.

(Cherry-picked commit 0b72cf5c4eca0e0db76e3e210cfdc48e6b49bb74 from PR#1825)

install.d/50-dracut.install

@@ -4,6 +4,7 @@ COMMAND="$1"
 KERNEL_VERSION="$2"
 BOOT_DIR_ABS="$3"
 KERNEL_IMAGE="$4"
+INITRD_OPTIONS_SHIFT=4
 
 # If KERNEL_INSTALL_MACHINE_ID is defined but empty, BOOT_DIR_ABS is a fake directory.
 # In this case, do not create the initrd.
@@ -34,6 +35,9 @@ ret=0
 
 case "$COMMAND" in
     add)
+        # If the initrd was provided on the kernel command line, we shouldn't generate our own.
+        [ "$#" -gt "$INITRD_OPTIONS_SHIFT" ] && exit 0
+
         if [[ $IMAGE == "uki.efi" ]]; then
             IMAGE_PREGENERATED=${KERNEL_IMAGE%/*}/uki.efi
         else