stm32mp1: dynamically build DFU_ALT_INFO

This patch reduces the stm32mp1 environment size and
builds dynamically the DFU board configuration with gpt
and mtd partitions and information from defconfig
(CONFIG_DFU_ALT_RAM0).

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