fix(make_chroot): Improve chroot user creation.

If the user already exists check that the UID and GID are correct and
modify it (setting shell and home directory) to match what the SDK
expects. This avoids needlessly failing if the user calling cros_sdk is
the 'core' user on a CoreOS machine.

Change new-user creation to copy the user's full name and group instead
of using a generic name and Google's 'eng' group. Also remove the
default password for the account, it isn't needed and uses perl.
fix(build_image): Add support for using git in dev images.

If git is installed via coreos-dev in the STATE partition it will need
some help finding its install location since it was built thinking it
would be installed in /usr rather than /usr/local.
Revert "fix(build_library): Create disk image first instead of last."

This reverts commit b97cfe126f0934d1505e352e17f15d580879d3cc.

The minor device numbers of loop partitions are allocated dynamically
which significantly complicates dunning under Docker which uses a static
/dev. Rolling this back until we can rely on /dev being dynamic.