Don't generate the rootfs every time when cross-compiling

[SteveDesmond-ca]

Steps to reproduce:

1. sudo ./build.sh /p:Platform=arm
2. let it build all the way
3. sudo ./build.sh /p:Platform=arm again

Expected result:

• Not much should happen, since it just got build and nothing changed

Observed result:

• The rootfs is regenerated right off the bat, which takes a really long time and downloads an entire OS

We should add a condition to only generate the rootfs if it doesn't already exist.

[eerhardt]

@weshaggard is currently fixing this in #158

[weshaggard]

Fixed by #158