recovery: change the way of rebooting when using power key combo

The power key combo allow to reboot from recovery mode by pressing
power button 7 times in a row. It calls directly the function
android_reboot() and lead to permission denial errors because of SE
Linux rules enforcement.
The right way to reboot from recovery is to set the property
"sys.powerctl" and let init handle it.

Change-Id: Ic7b81e446c3ee13dfbad10cda13a6a1f93123b76
Signed-off-by: Gaelle Nassiet <gaellex.nassiet@intel.com>