build-all.py: Update defconfigs with savedefconfig

Copy the savedefconfig (or minimal defconfig) instead of the
.config when updating defconfigs.

Change-Id: I0ba0b7a774dd3de0dc090420bd5c118eb6199eb6
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
(cherry picked from commit 6575c0b29f060a5ebdeec0ff9bcfe0f1af9f0bc8)