package/mender: new package

package/mender: new package

This patch add mender, an open source over-the-air (OTA) software
updater for embedded Linux devices.

Signed-off-by: Angelo Compagnucci <angelo@amarulasolutions.com>
[Thomas:
 - add entry to DEVELOPERS file.
 - drop dependency on systemd, since there is really no build
   dependency, it's just that the init script integration is missing.
 - add Config.in comment about the thread dependency
 - don't override install commands, otherwise the mender binary is not
   installed, and instead use a post install target hook.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>