board: stm32: fix extra env setings addresses

For stm32f4, f7 and h7 boards, reserve:
 - 4MB for kernel
 - 64KB for fdt, boot script, pxefile
 - the remaining memory for ramdisk

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