Add a .gitignore for the uncompressed rootfs.

This avoids this sort of message when running repo upload:

Uncommitted changes in kernel/tests (did you forget to amend?):
net/test/net_test.rootfs.20150203
Continue uploading? (y/N)

Change-Id: I4346f701e2b3197e52dabeeda2d0dc8307c037aa