u-boot: Add support to use uboot-extlinux-config class

u-boot: Add support to use uboot-extlinux-config class

Use uboot-extlinux-config class to create extlinux.conf file and then
install inside /boot/extlinux directory and also put file to deploy
dir. This file will be only create if UBOOT_EXTLINUX is set to 1.

You can use DEPLOYDIR/extlinux.conf file to install into final image
using wic setting:

IMAGE_BOOT_FILES_append = " extlinux.conf;extlinux/extlinux.conf"

Signed-off-by: Fabio Berton <fabio.berton@ossystems.com.br>
Signed-off-by: Otavio Salvador <otavio@ossystems.com.br>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>