env: mmc: introduced ENV_MMC_OFFSET

Introduce ENV_MMC_OFFSET defines.
It is a preliminary step to the next patches to simplify the code.

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