This commit is not breaking the current status, but adds the
ability to build on the host, to simulate and (mainly) to help
me hack the networking code in a simpler/faster environment.

Missing features:
	- ptp
	- build in i386 (currently x86-64 is hardwired)

Also, this commit requires a change in ppsi, that I'm not going to
commit now: we must get rid of the main function (most of that
stuff is done win wrc_ptp.c, which we must replicate  in a unix-compatible
way).

This is the ppsi change

   --- a/arch-unix/Makefile
   +++ b/arch-unix/Makefile
   @@ -4,7 +4,7 @@ A := arch-$(ARCH)

    CFLAGS += -Itools

   -OBJ-y += $A/unix-startup.o \
   +OBJ-y += \
           $A/main-loop.o \
           $A/unix-io.o \
           $A/unix-conf.o \

Local changes in this commit:
      - adding CONFIG_HOST_PROCESS and the auto-generated opposites:
      CONFIG_LM32 and CONFIG_EMBEDDED_NODE (LM32 && NODE)

      - make some config option depend on !HOST_PROCESS

      - Makefile: some dependencies on HOST_PROCESS and the opposite

      - */*.mk: some dev and shell objects depend on
      CONFIG_EMBEDDED_NODE instead of CONFIG_WR_NODE (i.e.: both a
      node and an lm32)

      - new host/ subdir where some placeholders are placed

      - no sdb support on the host

Signed-off-by: Alessandro Rubini <rubini@gnudd.com>