imx: mkimage_fit_atf: fix file size reporting

imx: mkimage_fit_atf: fix file size reporting

instead using ls and awk to determine file size use stat instead.
This fixes an invalid size reporting for user or group names that have
spaces in them.

This adds a dependency on the stat application which is part of
the coreutils package which also includes ls.

Signed-off-by: Tim Harvey <tharvey@gateworks.com>