Add debounce in thermal zone notifications

Sometimes we get too may notification via kobject uevent.
Adding a 3 seconds debounce interval.
Replace hardcoded max_sensors for zones

Using read_dir to scan through thermal zones and identifying
index for pkg_temp zone.
Upstart script: add sleep

When thermald is launched during system startup, coretemp and thermal
zone sysfs is not ready and read fails. Added a 5 seconds sleep,
giving sufficient time for coretemp and thermal sysfs to be ready.