board: stm32mp1: Add env_ext4_get_dev_part() and env_ext4_get_intf()

This allows to :
- select the current device to save the environment file
- select the correct EXT4 boot device instance
  and partition to save the environment file.

For EXT4, device is mmc, device instance is 0 for sdcard or 1 for eMMC.
The partition is set to "auto" to select the first partition with
bootable flag.

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