PKG_BUILD_DIR was defined with quoting PKG_VERSION in
layerscape package makefiles. Now PKG_VERSION has been
removed from these makefiles. When PKG_BUILD_DIR quotes
PKG_VERSION, '=' should be used instead ':=' to make
sure PKG_VERSION has been defined in common makefile.
Signed-off-by: Yangbo Lu <yangbo.lu@nxp.com>