linux-yocto: add vfat KERNEL_FEATURE when MACHINE_FEATURES include vfat

If a machine has vfat in its MACHINE_FEATURES, we can ensure that the
kernel has support by enabling the associated KERNEL_FEATURE.

This partially addresses Yocto bug 14470.

Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>