stm32mp1: imply CMD_CLS

stm32mp1: imply CMD_CLS

Activate by default the command CLS (clear screen);
this command used in pxe or sysboot command (DISTRO support)
when the "menu background" keyword is present.

This patch avoid the warning "Unknown command 'cls'"
with extlinux.conf:

# Generic Distro Configuration file generated by OpenEmbedded
menu title Select the boot mode
MENU BACKGROUND /splash.bmp
TIMEOUT 20
DEFAULT stm32mp157c-ev1-emmc
LABEL stm32mp157c-ev1-emmc
	KERNEL /uImage
	FDT /stm32mp157c-ev1.dtb
	APPEND root=/dev/mmcblk1p4 rootwait rw console=ttySTM0,115200

  ...
  Retrieving file: /mmc0_stm32mp157c-ev1_extlinux/extlinux.conf
  614 bytes read in 36 ms (16.6 KiB/s)
  Retrieving file: /splash.bmp
  46180 bytes read in 40 ms (1.1 MiB/s)
  Unknown command 'cls' - try 'help'
  Select the boot mode
  1:	stm32mp157c-ev1-sdcard
  ...

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Acked-by: Patrick Delaunay <patrick.delaunay@st.com>