build_rootfs.sh: fix e2fsck command not found error We need to add the full path when running e2fsck as a user because sometimes /sbin is not in PATH. Change-Id: I06f6c744d0899ad5105f10aeed7451afeb0a7ec3 Signed-off-by: Ying-Chun Liu (PaulLiu) <paul.liu@linaro.org>