tools/examples: cleanup Makefile

tools/examples: cleanup Makefile

Don't check if a target exist before installing it. For directory,
install doesn't complain, and for file it would prevent from updating
them. Also remove the existing loop and instead install all files with
a single call to $(INSTALL_DATA).

Remove XEN_CONFIGS-y which isn't used.

Remove "build" target.

Add an empty line after the first comment. The comment isn't about
$(XEN_READMES), it is about the makefile as a whole.

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