build-deps: Add device-tree-compiler

Add the device-tree-compiler so that Chromebook images will be able to run mkimage properly

Signed-off-by: Steev Klimaszewski <steev@gentoo.org>
all: Don't compress the images on 32bit.

pixz seems to have issues compressing the image files on 32bit systems.
Since we will always build official images on 64bit, only set it to compress and sha1sum the compressed file on 64bit.

Signed-off-by: Steev Klimaszewski <steev@gentoo.org>