cmd: pxe_utils: sysboot: replace cls command by video_clear in PXE parser

Since the commit bfaa51dd ("cmd: add serial console support
for the cls command") the cls command is not enough to clear the
video display when ANSI console is activated.

This patch clears the video device with the video_clear() API
before to display the bitmap used for the PXE background.

This patch avoids to display the LOGO, activated by default with
commit 7a8555d8 ("video: Show the U-Boot logo by default").

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@foss.st.com>