boot_fat.mk: Rework boot partition creation to use mtools

Original Work done by John Stultz.

Use mtools rather then mounting the partition, in order
to avoid requiring root permissions to build.

Cc: Vishal Bhoj <vishal.bhoj@linaro.org>
Cc: Guodong Xu <guodong.xu@linaro.org>
Signed-off-by: John Stultz <john.stultz@linaro.org>
Signed-off-by: Vishal Bhoj <vishal.bhoj@linaro.org>
1 file changed