makefile: drop PREFIX, comply with /usr merge

PREFIX was dropped and paths updated so that they
were compliant with a merged /usr layout.

Makefile

 install:
 	install -Dm 755 src/swclock-offset-boot.sh \
-		$(DESTDIR)$(PREFIX)/sbin/swclock-offset-boot
+		$(DESTDIR)/usr/bin/swclock-offset-boot
 	install -Dm 755 src/swclock-offset-shutdown.sh \
-		$(DESTDIR)$(PREFIX)/sbin/swclock-offset-shutdown
+		$(DESTDIR)/usr/bin/swclock-offset-shutdown
 
 install_openrc:
 	install -Dm 755 openrc/swclock-offset-boot.initd \
@@ -12,6 +12,6 @@ install_openrc:
 
 install_systemd:
 	install -Dm 644 systemd/swclock-offset-boot.service \
-		$(DESTDIR)/lib/systemd/system/swclock-offset-boot.service
+		$(DESTDIR)/usr/lib/systemd/system/swclock-offset-boot.service
 	install -Dm 644 systemd/swclock-offset-shutdown.service \
-		$(DESTDIR)/lib/systemd/system/swclock-offset-shutdown.service
+		$(DESTDIR)/usr/lib/systemd/system/swclock-offset-shutdown.service