thermal/tools/tmon: Improve the Makefile

 - Remove empty macros assignments
 - Use directory creation parameter for the install tool
 - Use $OBJ instead of building the list of object for the 'clean' target

[dlezcano] : Changed title and description

Signed-off-by: Rolf Eike Beer <eb@emlix.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1951386.ZPQrlMDjM2@devpool47