bcm53xx: fix target name of meraki_mx64-a0

bcm53xx: fix target name of meraki_mx64-a0

The target name of meraki_mx64-a0 in
target/linux/bcm53xx/image/Makefile used not to be consistent with the
one defined in target/linux/bcm53xx/base-files/lib/upgrade/platform.sh
and generates warning for "Image check failed" during sysupgrade.

This commit would also make the target name for meraki_mx64-a0 to
conform to the openwrt standard.

Signed-off-by: Edward Chow <equu@openmail.cc>
Link: https://github.com/openwrt/openwrt/pull/22034
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>