board/freescale: increase the vfat size

The default iamge size is 32MiB, which is quite low by today's standards.
Besides, the AArch64 kernels are relatively big, which leaves not much
room, if at all, for users to experiment on the default image.

Increase the vfat size to a more reasonable 64MiB.

Note that users who derive an in-tree defconfig for their own case will
allways hit any arbitarary size we put here, so they will anyway have to
also derive this template for their own use-cases.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>