configs: stm32f746-disco: update EXTRA_ENV_SETTINGS

Allows to boot linux distribution.

As stm32f746-disco and stm32f769-disco are sharing the same defconfig
file, in case of stm32f769-disco build, "fdtfile=..." must be updated
with the correct stm32f769 DTB file name.

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