kernel-install: respect $MACHINE_ID and ignore /etc/machine-id if on tmpfs

Confer https://github.com/systemd/systemd/pull/19006#issuecomment-800234022:
  On some systems it's the admin's explicit choice not to to have the
  machine ID leak into the ESP
  On some systems the machine ID is transient, generated at every boot,
  and hence should not be written to the ESP

src/kernel-install/kernel-install

@@ -85,20 +85,14 @@ fi
 KERNEL_VERSION="$1"
 KERNEL_IMAGE="$2"
 
-# Reuse directory created without a machine ID present if it exists.
-if [[ -d /efi/Default ]] || [[ -d /boot/Default ]] || [[ -d /boot/efi/Default ]]; then
-    MACHINE_ID="Default"
-elif [[ -f /etc/machine-id ]]; then
-    read MACHINE_ID < /etc/machine-id
-else
-    MACHINE_ID="Default"
-fi
-
 if [[ ! $COMMAND ]] || [[ ! $KERNEL_VERSION ]]; then
     echo "Not enough arguments" >&2
     exit 1
 fi
 
+[ -z "$MACHINE_ID" ] && [ -f /etc/machine-id ] && [ "$(stat -fc %T /etc/machine-id)" != "tmpfs" ] && read -r MACHINE_ID < /etc/machine-id
+[ -z "$MACHINE_ID" ] && MACHINE_ID="Default"
+
 if [[ -d /efi/loader/entries ]] || [[ -d /efi/$MACHINE_ID ]]; then
     ENTRY_DIR_ABS="/efi/$MACHINE_ID/$KERNEL_VERSION"
 elif [[ -d /boot/loader/entries ]] || [[ -d /boot/$MACHINE_ID ]]; then
@@ -113,7 +107,7 @@ else
     ENTRY_DIR_ABS="/boot/$MACHINE_ID/$KERNEL_VERSION"
 fi
 
-export KERNEL_INSTALL_MACHINE_ID=$MACHINE_ID
+export KERNEL_INSTALL_MACHINE_ID="$MACHINE_ID"
 
 ret=0