lantiq: fix xway image building

lantiq: fix xway image building

Commit 8c130dd (lantiq: use new image build process for some images) broke
image generation for the xway target since the generated macros reference not
existing WBMRA.dts and WBMRB.dts files leading to the following build error:

  mips-openwrt-linux-musl-cpp: error: ../dts/WBMRA.dts: No such file or directory
  mips-openwrt-linux-musl-cpp: warning: '-x assembler-with-cpp' after last input file has no effect
  mips-openwrt-linux-musl-cpp: fatal error: no input files

Previously both the WBMRA and WBMRB profiles shared a common WBMR DT since
the only difference is the embedded firmware kmod package.

Extend the lantiqImage macro to optionally support specifying a different
DTS file and use this facility to let the both WBMR profiles reference the
correct device tree file.

Signed-off-by: Jo-Philipp Wich <jo@mein.io>