fsl-layerscape: fix warning if no hwconfig is defined

While getting the 'subarg' of 'hwconfig' env variable in
config_core_prefetch(), if no hwconfig variable is defined,
below warning is received:
WARNING: Calling __hwconfig without a buffer and
	before environment is ready

Fix this by checking 'hwconfig' env variable.
If not found return without further processing.

Signed-off-by: Pankaj Bansal <pankaj.bansal@nxp.com>
Signed-off-by: Priyanka Jain <priyanka.jain@nxp.com>
Tested-by: Michael Walle <michael@walle.cc>