postinst: Update postinstall_example documentation.

We don't need the /postinst symlink anymore, instead we should provide
the path to the executable in the config. An example of how to setup
that is now included in the documentation of postinstall_example.

Bug: 27177071
TEST=Deployed this script as postinstall on edison-eng.

Change-Id: I2f7bd50174db42a57d25277883b0ec795ca17144
2 files changed