riscv: add trampoline in MBR block to support boot mode 1

Add "j pc + 0x0800" at the beginning of the MBR to jump to bootblock.

Tested on hardware:
boot mode 15: works as before
boot mode 1: jump to bootblock works, but bootblock needs to be modified to
move the stack to L2LIM. This will be in a separate commit.

Further changes are needed in the bootblock

Change-Id: I16e762d9f027346b124412f1f7ee6ff37f431d86
Signed-off-by: Philipp Hug <philipp@hug.cx>
Reviewed-on: https://review.coreboot.org/27397
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
2 files changed