rtc-tools: Add a Makefile

Add a Makefile to make installation and uninstallation
process easier.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20220127225928.34214-1-festevam@gmail.com