uboot: add COPY_OLD_LICENSE_FILE fix to rsync hook

make legal-info breaks when using an older u-boot version in local.mk
with OVERRIDE_SRCDIR, due to the reorganization of license files which
happened in u-boot v2013.10.
This issue had already been fixed in Buildroot for the regular use of
old u-boot versions (non-local.mk).
This patch adds the same fix to UBOOT_POST_RSYNC_HOOKS so that it also
covers the local.mk case.

Signed-off-by: Erico Nunes <nunes.erico@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>