busybox: Disable mkfs.vfat from busybox

'dosfstools' is providing 'mkfs.vfat' so no need to take it from
busybox.

Signed-off-by: Muhammad Shakeel <muhammad_shakeel@mentor.com>
Merge pull request #38 from mshakeel/busybox

busybox: Disable mkfs.vfat from busybox