fix(base): support readlink applet from busybox

busybox's `readlink` applet does not support the option `-e` option.

Replace these `readlink` calls by `realpath`. Since `realpath` will
return successfully on non-existing path, check the path exists if
needed.

Part of: #2437

Author: bdrung

modules.d/80base/dracut-lib.sh

@@ -497,7 +497,7 @@ udevproperty() {
 
 find_mount() {
     local dev wanted_dev
-    wanted_dev="$(readlink -e -q "$1")"
+    wanted_dev="$(realpath "$1")"
     while read -r dev _ || [ -n "$dev" ]; do
         [ "$dev" = "$wanted_dev" ] && echo "$dev" && return 0
     done < /proc/mounts
@@ -628,7 +628,7 @@ copytree() {
     local src="$1" dest="$2"
     [ -d "$src" ] || return 1
     mkdir -p "$dest" || return 1
-    dest=$(readlink -e -q "$dest") || return 1
+    dest=$(realpath "$dest") || return 1
     (
         cd "$src" || exit 1
         cp -af . -t "$dest"
@@ -710,8 +710,9 @@ devnames() {
     esac
 
     for d in $dev; do
+        [ -e "$d" ] || return 255
         names="$names
-$(readlink -e -q "$d")" || return 255
+$(realpath "$d")"
     done
 
     echo "${names#