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>