uboot-layerscape-armv8_32b: fix package download

uboot-layerscape-armv8_32b: fix package download

In commit 2b1ec44dbd3cd3 ("layerscape: add ls1012afrdm device support")
The git revision and the mirror hash for this package was updated to a
version which includes ls1012afrdm-uboot.bin, but the file name at
dl/uboot-layerscape-armv8_32b-2017.09.tar.xz staid the same. This way
most user did not download the new version but used the old file.
Convert this package to the normal git clone parameters by using
PKG_SOURCE_DATE instated of PKG_VERSION, now the file name in dl also
contains the git hash and should change every time the git hash is
updated.

This should fix a problem spotted by build bot.

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