configs: stm32mp1: set the console variable for extlinux.conf

Set the environment variable "console", used in extlinux.conf file when it
is generated by YOCTO distribution with:

UBOOT_EXTLINUX_CONSOLE ??= "console=${console},${baudrate}"

With these 2 variables, U-Boot give dynamically the used console and
baudrate in the Linux kernel bootargs.

For the STMicroelectronics boards, the used console is ttySTM0.

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