initramfs: use busybox mountpoint rather than copying /bin/mountpoint

/bin/mountpoint was copied to the initramfs only when --multipath
is enabled. Now, I'm not sure why we need the full blown mountpoint
when there is busybox mountpoint which exposes the same cmdline
interface. Furthermore, bb mountpoint can be used to determine if
a filesystem is already mounted.

gen_initramfs.sh

@@ -131,7 +131,7 @@ append_busybox() {
 
 	# Set up a few default symlinks
 	local default_applets="[ ash sh mount uname ls echo cut cat flock stty"
-	default_applets+=" readlink"
+	default_applets+=" readlink mountpoint"
 	for i in ${BUSYBOX_APPLETS:-${default_applets}}; do
 		rm -f ${TEMP}/initramfs-busybox-temp/bin/$i
 		ln -s busybox ${TEMP}/initramfs-busybox-temp/bin/$i ||
@@ -206,7 +206,6 @@ append_multipath(){
 
 	# Copy files
 	copy_binaries "${TEMP}/initramfs-multipath-temp" \
-		/bin/mountpoint \
 		/sbin/{multipath,kpartx,mpath_prio_*,devmap_name,dmsetup} \
 		/{lib,lib64}/{udev/scsi_id,multipath/*so}