commands: Correct the operation of the unified build check

The current code reads the config file and the fdtget tool from the wrong
places (they are swapped). Adjust this to behave correctly:

   - config.dtb should come from the board's root inside the chroot
   - fdtget should come from the chroot itself

Tidy up the function comment a little also to make it clearer.

BUG=chromium:747023
BUG=chromium:704597
TEST=PYTHONPATH=~/cosarm ./commands_unittest
PYTHONPATH=~/cosarm ./build_stages_unittest
Also manually print out a few things as the test is not very good

Change-Id: If54a1e7165bca07f10d2d789992ccba95c78b8ff
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-on: https://chromium-review.googlesource.com/579792
Reviewed-by: Jason Clinton <jclinton@chromium.org>
3 files changed