tools: build golang tools if go compiler is present

tools: build golang tools if go compiler is present

By default, if the go compiler is found by the configure script, build
the golang tools. If the compiler is not found, and --enable-golang was
not explicitly set, do not build to the golang tools.

The corresponding make variable is CONFIG_GOLANG. Remove CONFIG_GOLANG
from tools/Rules.mk since the variable is now set by configure in
config/Tools.mk.

Signed-off-by: Nick Rosbrook <rosbrookn@ainfosec.com>
Acked-by: Wei Liu <wl@xen.org>
Acked-by: George Dunlap <george.dunlap@citrix.com>