Rework the evk boot flow to use the new piggydata load function and
install a trampoline for the TF-A setup. This allows the PBL boot
process to stay in SRAM up until the verification of the piggydata is
done and main barebox can be loaded.

The trampoline loads 4 bytes right after the trampoline, we copy the
runtime offset there so the trampoline jumps back into the SRAM PBL.

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