stm32mp: stm32prog: add support of boot partition for eMMC device

Add support of eMMC device boot partition with
part_id = -1 for offset="boot1"
     or = -2 for offset="boot2"

The stm32prog command configures the MMC DFU backend with "mmcpart"
and configure the eMMC (command "mmc bootbus" and "mmc partconf")
when the update is done.

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