MIPS: INCA-IP: rename inca-swap-bytes host tool

The INCA-IP SoC belongs to the Lantiq XWAY SoC product portfolio.
For the upcoming support of other Lantiq SoC devices this tool should
not solely depend on the INCA-IP board.

Rename the tool to xway-swap-bytes and add an config option
to enable compilation optionally.

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@googlemail.com>
Acked-by: Thomas Langer <thomas.langer@lantiq.com>
Signed-off-by: Shinya Kuribayashi <skuribay@pobox.com>