tools/xentop: rework makefile

tools/xentop: rework makefile

Add "xentop" to "TARGETS" because this variable will be useful later.

Always define all the targets, even when configured with
--disable-monitor, instead don't visit the subdirectory.
This mean xentop/ isn't visited anymore during "make clean" that's how
most other subdirs in the tools/ works.

Also add missing "xentop" rules. It only works without it because we
still have make's built-ins rules and variables, but fix this to not
have to rely on them.

Use $(TARGETS) with $(INSTALL_PROG), and thus install into the
directory rather than spelling the program name.

In the "clean" rule, use $(RM) and remove all "*.o" instead of just
one object.

Signed-off-by: Anthony PERARD <anthony.perard@citrix.com>
Reviewed-by: Luca Fancellu <luca.fancellu@arm.com>