hal/dev_mngr/dev_mngr_core.c: dmngr_scan_devs () is now idempotent

We can call dmngr_scan_devs () as many times we want to search
for new devices and then call for the initialization function
for each device found safely.

This fixes #41 github issue.
 hal/dev_mngr/dev_mngr.c: add dmngr_scan_devs () to main loop

 This allow us with a more dynamic device initialization.