hwconfig: Fix handling of env_hwconfig, board_hwconfig, and cpu_hwconfig

hwconfig: Fix handling of env_hwconfig, board_hwconfig, and cpu_hwconfig

The handling of env_hwconfig, board_hwconfig, and cpu_hwconfig got
broken when we removed the boards defining dummy board_hwconfig
& cpu_hwconfig values.

We fix this by handling the various strings in priority order.  If
hwconfig_parse returns NULL for a given string we check the next one
in order (env_hwconfig, board_hwconfig, followed by cpu_hwconfig).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>