init: Add support for writing system property value to a file in init.rc

The write command will write a property value if the value starts with a "$'

For example:
    write /sys/class/android_usb/iSerial $ro.serialno

Use double leading '$' if you need to write a string that starts with '$':

    write /data/foo $$hello

to write "$hello" to /data/foo

Change-Id: I55431ac7715a5347bb95c3a15aee97c575444dde