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]

include/fastboot.h

@@ -38,6 +38,9 @@ enum {
 #if CONFIG_IS_ENABLED(FASTBOOT_CMD_OEM_FORMAT)
 	FASTBOOT_COMMAND_OEM_FORMAT,
 #endif
+#if CONFIG_IS_ENABLED(FASTBOOT_CMD_OEM_PARTCONF)
+	FASTBOOT_COMMAND_OEM_PARTCONF,
+#endif
 
 	FASTBOOT_COMMAND_COUNT
 };