boards: arm: nucleo_f030r8: use smaller board image

At the moment we have different images for
for Nucleo F030R8 and Nucleo F070RB boards,
the images have the same pixel size but different
file formats, e.g:

  NAMES="f030r8 f070rb"
  for i in $NAMES; do
    file boards/arm/nucleo_$i/doc/img/nucleo_$i.jpg;
  done

  boards/arm/nucleo_f030r8/doc/img/nucleo_f030r8.jpg: JPEG image
  data, Exif standard: [TIFF image data, little-endian,
  direntries=0], baseline, precision 8, 500x367, frames 3
  boards/arm/nucleo_f070rb/doc/img/nucleo_f070rb.jpg: JPEG image
  data, JFIF standard 1.01, aspect ratio, density 1x1,
  segment length 16, progressive, precision 8, 500x367, frames 3

The nucleo_f030r8.jpg file is larger:

  for i in $NAMES; do
    ls -1 -sh boards/arm/nucleo_$i/doc/img/nucleo_$i.jpg;
  done

  128K boards/arm/nucleo_f030r8/doc/img/nucleo_f030r8.jpg
  40K boards/arm/nucleo_f070rb/doc/img/nucleo_f070rb.jpg

Applying simultaneous black/white threshold to the images
and comparing them with imagemagick tools shows that
the images have no significant difference.

  for i in $NAMES; do
    convert boards/arm/nucleo_$i/doc/img/nucleo_$i.jpg \
            -threshold 80% /tmp/$i.png;
  done
  compare $(for i in $NAMES; do echo -n  "/tmp/$i.png "; done) \
            -compose src /tmp/diff.png

See also 'boards: arm: unify Nucleo-64 boards connectors image'
(https://github.com/zephyrproject-rtos/zephyr/pull/15926

).

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>