YAY! kernel post install script to update /boot/uEnv.txt

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
chroot: add /boot/uEnv.txt post install script

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
chroot: set correct permissions for: /etc/kernel/postinst.d/zz-uenv_txt

Signed-off-by: Robert Nelson <robertcnelson@gmail.com>