Check for /dev/ in findmnt results for source /boot and root

There can be multiple findmnt lines (eg systemd and /dev/...) for /boot
on some systems and the device /dev entry is the one needed.
So grep for /dev/ on those findmnt results.

README.md

@@ -1,5 +1,5 @@
 ## rpi-clone
-Latest version: 2.0.21
+Latest version: 2.0.22
 
 Version 2 is a complete rewrite with improved capability over
 the original.  See the examples below.
@@ -111,6 +111,7 @@ usage: sys-clone sdN {-v|--verbose} {-f|--force-initialize} {-f2}
          {-L|--label-partitions label} {-l|--leave-sd-usb-boot}
          {-a|--all-sync} {-F|--Force-sync} {-x} {-V|--version}
          {--convert-fstab-to-partuuid}
+         {--exclude=PATTERN} {--exclude-from=FILE}
 
     -v	    - verbose rsync, list all files as they are copied.
     -f	    - force initialize the destination disk by imaging the booted disk
@@ -137,8 +138,8 @@ usage: sys-clone sdN {-v|--verbose} {-f|--force-initialize} {-f2}
                 the custom list will be synced instead of the default of all
                 mounted directories.  The root directory is always synced.
                 Not for when initializing.
-    -L lbl  - label for ext type partitions.  If 'lbl' ends with #, replace
-				the # with a partition number and label all ext partitions.
+    -L lbl  - label for ext type partitions.  If 'lbl' ends with '#', replace
+				the '#' with a partition number and label all ext partitions.
                 Otherwise apply label to root partition only.
     -l      - leave SD card to USB boot alone when cloning to SD card mmcblk0
                 from a USB boot.  This preserves a SD card to USB boot setup

rpi-clone

@@ -8,7 +8,7 @@
 #    https://github.com/billw2/rpi-clone
 
 
-version=2.0.21
+version=2.0.22
 
 # auto run grub-install if grub detected
 grub_auto=1
@@ -583,8 +583,8 @@ get_src_disk()
 
 # ==== source (booted) disk info and default mount list
 #
-src_boot_dev=`findmnt /boot -o source -n`
-src_root_dev=`findmnt / -o source -n`
+src_boot_dev=`findmnt /boot -o source -n | grep "/dev/"`
+src_root_dev=`findmnt / -o source -n | grep "/dev/"`
 SD_slot_boot=0
 SD_slot_dst=0
 src_part_base=""