Cubietruck and other fixes

Created by: SunilMohanAdapa

The patchset fixes the following issues:

• When output directory is absolute path, writing u-boot firmware to image fails.
• Use flash-kernel for A20 based devices since it seems to work fine.
• Remove subvol=@ root file system parameter so that we work properly with upstream vmdebootstrap.

I have tested all the above by building a Cubietruck image with this branch.