vold: add property hook to allow ignore voldmanaged devices

devices like raspi can boot from usb but also want other usb
devices as voldmanaged. In that case the boot devices should
be ignored

Use property sys.vendor.vold.skip_root_mnt=true
then this device will be ignored from the voldmanaged
list

Change-Id: I1980884addee1d773e9b3950ba77e043c2ee6417
1 file changed