aemfvp-a: Populate ext4 with mkfs.ext4 -d

Instead of modifying the file-system after creation, make use of the
mkfs.ext4 -d parameter to populate the file-system at creation.

Co-authored-by: Patrik Berglund <patrik.berglund@arm.com>
Co-authored-by: Ben Taylor <ben.taylor@linaro.org>
Signed-off-by: Ben Taylor <ben.taylor@linaro.org>
Change-Id: Icdfff8bdf169472b6ccaa566d4e6d2f64e59460c