ath79: lzma-loader: fix & re-enable per-board CONFIG_BOARD_DEV

ath79: lzma-loader: fix & re-enable per-board CONFIG_BOARD_DEV

Back in the AR71XX days, the lzma-loader code could be customized
based on the $BOARD variable. These would be passed as a
compile-time -DCONFIG_BOARD_$DEVICE_MODEL flag to the compiler.
Hence, the lzma-loader would be able to include device-specific
fixups.

Note: There's still a fixup for the TpLink TL-WR1043ND V1 found
in the lzma-loader's board.c code. But since the days of AR71XX
I couldn't find a forum post or bug reported. So, I left it
as is to not break anything by enabling it.

=> If you have a TL-WR1043ND V1 and you have problem with
the ethernet: let me know. Because otherwise, the fixup
might simply no longer needed with ath79 and it can be removed.

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>