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]
diff --git a/drivers/fastboot/Kconfig b/drivers/fastboot/Kconfig
index 8a92e75..1bcc8d4 100644
--- a/drivers/fastboot/Kconfig
+++ b/drivers/fastboot/Kconfig
@@ -189,6 +189,13 @@
 	  relies on the env variable partitions to contain the list of
 	  partitions as required by the gpt command.
 
+config FASTBOOT_CMD_OEM_PARTCONF
+	bool "Enable the 'oem partconf' command"
+	depends on FASTBOOT_FLASH_MMC && SUPPORT_EMMC_BOOT
+	help
+	  Add support for the "oem partconf" command from a client. This set
+	  the mmc boot-partition for the selecting eMMC device.
+
 endif # FASTBOOT
 
 endmenu