net: wan: wanxl: refactor the firmware rebuild rule

Split the big recipe into 3 stages: compile, link, and hexdump.

After this commit, the build log with CONFIG_WANXL_BUILD_FIRMWARE
will look like this:

  M68KAS  drivers/net/wan/wanxlfw.o
  M68KLD  drivers/net/wan/wanxlfw.bin
  BLDFW   drivers/net/wan/wanxlfw.inc
  CC [M]  drivers/net/wan/wanxl.o

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>