configs: stm32mp15: Enable ENV_IS_IN_EXT4 and all relative flags

Enable ENV_IS_IN_EXT4 and all relative flags to be able to
load/save environment in EXT4 partition.

This will allows to load/save environment on both sdcard and eMMC.
As for stm32mp15, bootfs has not the same partition number on sdcard
and on eMMC, we use "auto" key which allows to find the first
partition in device with bootable flag which is partition 4 on sdcard
and partition 2 on eMMC.

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