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>

Documentation/mainboard/sifive/hifive-unleashed.md

@@ -11,7 +11,7 @@ For general setup instructions, please refer to the [Getting Started Guide].
 
 The following things are still missing from this coreboot port:
 
-- Trampoline in the MBR block to support boot mode 1
+- Support running romstage from flash (fix stack) to support boot mode 1
 - CBMEM support
 - FU540 clock configuration
 - FU540 RAM init

util/riscv/sifive-gpt.py

@@ -25,6 +25,12 @@ BLOCK_MASK = BLOCK_SIZE - 1
 # Size of the bootcode part of the MBR
 MBR_BOOTCODE_SIZE = 0x1be
 
+# MBR trampoline to bootblock
+MBR_BOOTCODE = bytes([
+    # j pc + 0x0800
+    0x6f, 0x00, 0x10, 0x00,
+])
+
 # A protecive MBR, without the bootcode part
 PROTECTIVE_MBR_FOOTER = bytes([
     0x00, 0x00, 0x02, 0x00, 0xee, 0xff, 0xff, 0xff,
@@ -43,7 +49,7 @@ PROTECTIVE_MBR_FOOTER = bytes([
 # [1]: https://en.wikipedia.org/wiki/GUID_Partition_Table#PROTECTIVE-MBR
 class ProtectiveMBR:
     def __init__(self):
-        self.bootcode = bytes(MBR_BOOTCODE_SIZE)
+        self.bootcode = MBR_BOOTCODE + bytes(MBR_BOOTCODE_SIZE - len(MBR_BOOTCODE))
 
     def generate(self, stream):
         assert len(self.bootcode) == MBR_BOOTCODE_SIZE
@@ -177,5 +183,11 @@ if __name__ == '__main__':
 
     image.fixup()
 
+    # Verify if first partition is at expected lba, otherwise trampoline will
+    # fail
+    if image.partitions[0].first_lba != 4:
+        print('Warning: First partition not at expected location (LBA 4)')
+        sys.exit(1)
+
     with open(sys.argv[2], 'wb') as f:
         image.generate(f)