Enable native/bytecode compilation choices, and some Makefile simplification.

Enable native/bytecode compilation choices, and some Makefile simplification.

* Enable the independent bytecode compilation mode.

Previously this was unfortunately blocked by the hard wired xapi-client.cmxa in the Makefile. Now we should be able to load the bytecode version of xapi client lib in the OCaml interpreter for interactive development with instant feedback. Use $(COMPILE_NATIVE) and $(COMPILE_BYTE) to control these options.

* Change a few "XXX_TARGET = $(if $(equal $(XXX_CONF), yes), $(XXX_OBJ), none)" to "XXX_TARGET = $(if $(XXX_CONF), $(XXX_OBJ))"

The judge conditions are not strictly equivalent in semantics. The former holds only if XXX_CONF equals to "yes", the later condition holds when XXX_CONF is set to any non-negative string (not "no", "false", "0", none etc.). But I think it should be the same regarding XAPI's current Makefiles, and with more simplicity and tolerance. A lot more simplication and optimization can be done to these Makefiles, it's just a matter of time. Moreover XAPI should consider to update to new version of OMake (which should be better, but unfortunately not fully compatible with the old one).

Signed-off-by: Zheng Li <dev@zheng.li>