layerscape: update u-boot to LSDK-18.06

The u-boot source code had been migrated to codeaurora
for LSDK-18.06 release and the future release. This
patch is to update u-boot to LSDK-18.06 for both
uboot-layerscape and uboot-layerscape-armv8_32b packages.
Besides, this patch also introduced some other changes.
- Reworked uboot-layerscape makefile to make it more
  readable.
- Define package in uboot-layerscape-armv8_32b for each board.
- Fixed u-boot package selection in target image makefile.

Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>