scripts: fix successive runs of 'make debootsrap'
scripts: fix successive runs of 'make debootsrap'

During the first run of 'make debootstrap', the 'admin' user is created.
On a next run, the user 'admin' fails to be created... because it
already exists...
We now check if the user exists before creating it.

Fixes #2

sbxg/sbxg,sbxg/sbxg