dosfstools: add mkfs.vfat to ALTERNATIVE

The mkfs.vfat tool can also be provided by busybox via the CONFIG_MKFS_VFAT
configuration (not enabled by default in OE but can be enabled on
systems avoiding components based on GPLv3).

Signed-off-by: Ricardo Salveti <ricardo@foundries.io>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>