In the upcoming multi image build process we will cat images together.
To find the concatenated image we need to reliably find the end of the
current binary. This adds a dummy section at the end of a pbl binary.
Its only purpose is to mark the end of the image. The multi image
patches will add something to this section so that it doesn't get
discarded by the linker.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>