fastboot: add command to select the default emmc hwpart for boot

Add fastboot command oem partconf which executes the command
``mmc partconf <id> <arg> 0`` on the current <id> mmc device
to configure the eMMC boot partition with
<arg>: boot_ack boot_partition, so the command is:

$> fastboot oem partconf:<boot_ack> <boot_partition>

The partition_access argument is forced to 0 (userdata)

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
[lukma - Kconfig adjustments after merging this patch]