boot/uboot: copy IMX firmware files to uboot package dir

The mainline U-Boot can create an i.MX specific firmware image (e.g. flash.bin).
For this the i.MX firmware files (DDR, HDMI) must be in the toplevel directory.

Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
[yann.morin.1998@free.fr: simplify hook]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>