feat(squash): use busybox for early setup if available

Use busybox can help reduce the size of early setup environment.

With this change, everything is packed in the squash image, and
the setup files will be dropped once squash image setup is done,
so initramfs stage memory usage is reduced to the minimun,
and initramfs decompress is also faster.

File layout of a squash initramfs looks like this:

========================================================================
drwxr-xr-x   1 root     root            0 Feb 15 14:07 .
-rwxr-xr-x   1 root     root          946 Feb 15 14:07 init
lrwxrwxrwx   1 root     root            7 Feb 15 14:07 lib -> usr/lib
drwxr-xr-x   1 root     root            0 Feb 15 14:07 squash
-rw-r--r--   1 root     root     91000832 Feb 15 14:07 squash-root.img
drwxr-xr-x   1 root     root            0 Feb 15 14:07 usr
drwxr-xr-x   1 root     root            0 Feb 15 14:07 usr/bin
-rwxr-xr-x   1 root     root      1293688 Jul 27  2020 usr/bin/busybox
lrwxrwxrwx   1 root     root            7 Feb 15 14:07 usr/bin/echo -> busybox
lrwxrwxrwx   1 root     root            7 Feb 15 14:07 usr/bin/mkdir -> busybox
lrwxrwxrwx   1 root     root            7 Feb 15 14:07 usr/bin/modprobe -> busybox
lrwxrwxrwx   1 root     root            7 Feb 15 14:07 usr/bin/mount -> busybox
lrwxrwxrwx   1 root     root            7 Feb 15 14:07 usr/bin/sh -> busybox
lrwxrwxrwx   1 root     root            7 Feb 15 14:07 usr/bin/switch_root -> busybox
drwxr-xr-x   1 root     root            0 Feb 15 14:07 usr/lib
drwxr-xr-x   1 root     root            0 Feb 15 14:07 usr/lib/dracut
-rw-r--r--   1 root     root           23 Feb 15 14:07 usr/lib/dracut/build-parameter.txt
-rw-r--r--   1 root     root           31 Feb 15 14:07 usr/lib/dracut/dracut-051-93.git20210215.fc33
-rw-r--r--   1 root     root          358 Feb 15 14:07 usr/lib/dracut/modules.txt
-rw-r--r--   1 root     root            0 Feb 15 14:07 usr/lib/dracut/need-initqueue
drwxr-xr-x   1 root     root            0 Feb 15 14:07 usr/lib/modules
drwxr-xr-x   1 root     root            0 Feb 15 14:07 usr/lib/modules/5.10.11-200.fc33.x86_64
drwxr-xr-x   1 root     root            0 Feb 15 14:07 usr/lib/modules/5.10.11-200.fc33.x86_64/kernel
<... kernel module misc files skipped ... >
========================================================================

modules.d/99squash/module-setup.sh

@@ -19,6 +19,9 @@ depends() {
 }
 
 installpost() {
+    local _busybox
+    _busybox=$(find_binary busybox)
+
     # Move everything under $initdir except $squash_dir
     # itself into squash image
     for i in "$initdir"/*; do
@@ -37,7 +40,15 @@ installpost() {
     done
 
     # Install required modules and binaries for the squash image init script.
-    DRACUT_RESOLVE_DEPS=1 inst_multiple sh mount modprobe mkdir switch_root
+    if [[ $_busybox ]]; then
+        inst "$_busybox" /usr/bin/busybox
+        for _i in sh echo mount modprobe mkdir switch_root; do
+            ln_r /usr/bin/busybox /usr/bin/$_i
+        done
+    else
+        DRACUT_RESOLVE_DEPS=1 inst_multiple sh mount modprobe mkdir switch_root
+    fi
+
     hostonly="" instmods "loop" "squashfs" "overlay"
     dracut_kernel_post