Add a function to load and copy the piggy data to the correct offset
expected by barebox in the RAM. This way the PBL can later verify the
piggydata before uncompressing and loading the main barebox.

Signed-off-by: Rouven Czerwinski <r.czerwinski@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>