init: Allow wildcards in property triggers by using * for property value

For example, the following trigger will fire when the sys.foo property
is set to any value:

on property:sys.foo=*
    write /data/foo hello

It is also possible to refer to the property within the trigger actions:

on property:sys.foo=*
    write /data/foo $sys.foo

Change-Id: If78d20a532f77e17aa5703d53be581ad6736cbcf
Signed-off-by: Mike Lockwood <lockwood@android.com>