boot/uboot: fix hook to copy imx firmware files

Simplification has broken it. Fix it again.

Fixes: af99e7a5 ("boot/uboot: copy IMX firmware files to uboot package dir")

Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Heiko Thiery <heiko.thiery@gmail.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>