configs: stm32mp1: enable the fastboot oem command partconf

configs: stm32mp1: enable the fastboot oem command partconf

Enable the fastboot oem command partconf, used to select the correct
eMMC boot partition, with same format than 'mmc partconf'
with parameter: boot_ack boot_partition
On stm32mp1 family:
- boot_ack = 1 (Boot Acknowledge is needed by ROM code)
- boot_partition = 1 or 2 (Boot partition 1 / 2 enabled for boot)

So on EV1 board the expected commands to select boot partition 1 or 2
$> fastboot oem partconf:1 1
$> fastboot oem partconf:1 2

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