image_types: Restore pre-btrfs-tools 4.14.1 mkfs.btrfs shrink behavior

Currently the mkfs.btrfs generates large images with a lot of wasted
space. This happens since OE-core updated btrfs-tools from 4.13.3 to
4.15.1 in commit 94b645aa ("btrfs-tools: update to 4.15.1") .

Note in mkfs.btrfs(8) manpage section -r says the following:
"
  -r|--rootdir <rootdir>
...
       Note This option may enlarge the image or file to ensure
       it’s big enough to contain the files from rootdir. Since
       version 4.14.1 the filesystem size is not minimized. Please
       see option --shrink if you need that functionality.

  --shrink
     Shrink the filesystem to its minimal size, only works with
     --rootdir option.
...
       Note prior to version 4.14.1, the shrinking was done
       automatically.
"

Add the --shrink option to EXTRA_IMAGECMD_btrfs to reinstate the
original behavior and un-waste the space.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alexander Kanavin <alexander.kanavin@linux.intel.com>
Cc: Richard Purdie <richard.purdie@linuxfoundation.org>
Cc: Ross Burton <ross.burton@arm.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>