add check for ignored CONFIG_ENV_EXT4_DEVICE_AND_PART definition

Check whether user has explicitly defined device and partition where
environment file will be located before using 'auto' i.e. bootable
partition

Voids the need to set such partition as bootable to work with the
'dev:auto' tuple

Signed-off-by: Manuel Reis <mluis.reis@gmail.com>
Cc: Patrick Delaunay <patrick.delaunay@st.com>
Cc: Patrice Chotard <patrice.chotard@st.com>
Tested-by: Michael Opdenacker <michael.opdenacker@bootlin.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>