Grsecuirty - Ubuntu Kernel's .config Integration

I'm not 100% on how secure this this, but using your instructions without this step leaves our SecureDrop installation unable to decrypt our root partition. Anyway, I feel these instructions should include some support for encrypted partitions. Curious what you think about this pull request.

I added the part about rebooting the server so that the "uname -r" would work correctly. Seems like a good idea anyway.
Yeah, I added this because I thought you were still recommending full disk encryption. I see now that you are not: https://github.com/freedomofpress/securedrop/issues/511.

This change does indeed make the Ubuntu configuration the basis for make menuconfig. And no, I have not taken the time to determine exactly what needs to be enabled to get full disk encryption to work. (I might take the time to do that soonish.)

If you decide to reject this part of the pull request, I recommend adding a note to https://github.com/freedomofpress/securedrop/blob/develop/docs/ubuntu_config.md#partition-the-disks stating that the administrator might have trouble installing Grsecurity if they choose to fully encrypt their system. Or if you were planning to include a Grsecurity kernel in your repository, you might want to include the options for encrypted disks.