Pass debootstrap args to both invocations

- Both calls to debootstrap need to have lots of args, e.g. packages to
install.
- Move common arguments to preamble.mk
- Pass common argument variable to debootstrap.mk and rootfs.mk

Change-Id: I8d281f32ade1c8e744a4ad8c78a06156e11d55ba
3 files changed