postinst: Update postinstall_example.

The postinstall step now receives two parameters: the target slot and
the progress file descriptor. This file descriptor is a pipe where the
postinstall program can write back to the updater what's its current

Bug: 27880754
TEST=Deployed an image with the postinstall example installed.

Change-Id: I0afedb47556885d4e3b3f1b93be82314153ffc19
1 file changed