Add makefile in top directory

Makefile

+.PHONY: default xseg clean distclean build
+
+default: xseg
+
+build: xseg
+
+xseg:
+	make -C xseg
+	rm xseg/config.mk
+
+clean:
+	make -C xseg clean
+	rm xseg/config.mk
+
+install:
+	make -C xseg install
+	make -C xseg install-src
+	rm xseg/config.mk
+
+distclean:
+	make -C xseg distclean