layerscape: fix package download

layerscape: fix package download

The git hash was changed for multiple layerscape packages without
changing the version number. The LEDE build system will not download the
packages again if the old version is already there and so some people
and the build bots are using wrong version of some packages. Use
PKG_SOURCE_DATE instead of PKG_VERSION to generate packages with the
date and the first charterers of the git hash. This will change the file
name and make the build system download them again, also if in future
the git hash is changed the file name will change and trigger a new
download.

This should fix a problem spotted by build bot.

Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>