Fix flash and erase of eMMC Boot2 with Fastboot

The current U-Boot version has the next matches for boot partitions:
> mmc0boot0 to EMMC_BOOT1
> mmc0boot1 to EMMC_BOOT1 (should be EMMC_BOOT2)
This patch fixes a typo for the boot partition number.

Signed-off-by: Oleh Kravchenko <oleg@kaa.org.ua>
Cc: Pantelis Antoniou <panto@antoniou-consulting.com>
Cc: Marek Vasut <marex@denx.de>
Reviewed-by: Sean Anderson <sean.anderson@seco.com>