configs: stm32: Enable CONFIG_ENV_VARS_UBOOT_CONFIG

Enable CONFIG_ENV_VARS_UBOOT_CONFIG for all STM32 boards
It allows to retrieve the SoC name into the "soc" environment
variable.

Signed-off-by: Christophe Priouzeau <christophe.priouzeau@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Vikas Manocha <vikas.manocha@st.com>