board: stm32mp1: use IS_ENABLED to prevent ifdef in env functions

Use IS_ENABLED to prevent ifdef in env functions:
- env_get_location
- env_ext4_get_intf
- mmc_get_env_dev

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