cmd: pxe: Display splashscreen from extlinux.conf input

The objective is to provide a simple way to retrieve a BMP file,
and display it as splashscreen, from extlinux.conf file input.

For this, we take example on https://www.syslinux.org/wiki/


index.php?title=Menu#The_advanced_menu_system
and more particularly on MENU BACKGROUND chapter.
For this, add "menu background" support in pxe command.

As example, extlinux.conf content will look like:

# Generic Distro Configuration file generated by OpenEmbedded
menu title Select the boot mode
TIMEOUT 20
menu background ../splash.bmp
DEFAULT stm32mp157c-ev1-sdcard
LABEL stm32mp157c-ev1-sdcard
	KERNEL /uImage
	FDT /stm32mp157c-ev1.dtb
	APPEND root=/dev/mmcblk0p6 rootwait rw earlyprintk console=ttySTM0,115200

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>