config: Add explicit support for building on ESX.

config: Add explicit support for building on ESX.

The ESX userspace looks quite a bit like linux, but has some key
differences which need to be specially handled in the build.  To
distinguish between ESX and systems which use the linux datapath
module, this patch adds two new macros "ESX" and "LINUX_DATAPATH".
It uses these macros to disable building code on ESX which only
applies to a true Linux environment.  In addition, it adds a new
route-table-stub implementation which is required for the build to
complete successfully on ESX.

Signed-off-by: Ethan Jackson <ethan@nicira.com>