cmd: pxe: execute the cls command only when supported

cmd: pxe: execute the cls command only when supported

Execute the command cls (for clear screen), when the "menu background"
keyword is present in extlinux.conf file, only if the command is supported.

This patch avoid the warning "Unknown command 'cls'"
with "menu background" in extlinux.conf when CONFIG_CMD_BMP is activated
and CONFIG_CMD_CLS not activated (default for CONFIG_DM_VIDEO).

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