Set the bootargs in the MBR script.

src/scripts/build_gpt.sh

 #!/bin/bash
 #
 # Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 #
 # Load common constants.  This should be the first executable line.
 # The path to common.sh should be relative to your script's location.
 . "$(dirname "$0")/common.sh"
 
@@ skipping 94 matching lines @@
   # We need to know the location and size of the kernel so we can create the
   # U-Boot script to point to it. Let's create one fake GPT first which will
   # set the appropriate environment variables. Then we can create the correct
   # script and install it for real. A bit awkward, but this is only temporary.
   echo "Installing fake GPT first, to calculate locations..."
   install_gpt $OUTDEV $ROOTFS_IMG $KERNEL_IMG $STATEFUL_IMG /dev/zero
 
   # Create the U-Boot script to copy the kernel into memory and boot it.
   KERNEL_OFFSET=$(printf "0x%08x" ${START_KERN_A})
   KERNEL_SECS_HEX=$(printf "0x%08x" ${NUM_KERN_SECTORS})
+
+  BOOTARGS="root=/dev/mmcblk1p3"
+  BOOTARGS="${BOOTARGS} init=/sbin/init"
+  BOOTARGS="${BOOTARGS} console=ttySAC2,115200"
+  BOOTARGS="${BOOTARGS} mem=1024M"
+  BOOTARGS="${BOOTARGS} rootwait"
+
   MBR_SCRIPT="${IMAGEDIR}/mbr_script"
   echo -e "echo\necho ---- ChromeOS Boot ----\necho\n" \
+          "setenv bootargs ${BOOTARGS}\n" \
           "mmc read 1 C0008000 $KERNEL_OFFSET $KERNEL_SECS_HEX\n" \
           "bootm C0008000" > ${MBR_SCRIPT}
   MKIMAGE="${FLAGS_board_root}/u-boot/mkimage"
   if [[ -f "$MKIMAGE".gz ]]; then
     sudo gunzip "$MKIMAGE".gz
   fi
   if [[ -x "$MKIMAGE" ]]; then
     MBR_SCRIPT_UIMG="${MBR_SCRIPT}.uimg"
     "$MKIMAGE" -A "${ARCH}" -O linux  -T script -a 0 -e 0 -n "COS boot" \
                -d ${MBR_SCRIPT} ${MBR_SCRIPT_UIMG}
@@ skipping 31 matching lines @@
 echo "Copying kernel..."
 $sudo dd if=${KERNEL_IMG} of=${OUTDEV} conv=notrunc bs=512 seek=${START_KERN_A}
 
 echo "Copying rootfs..."
 $sudo dd if=${ROOTFS_IMG} of=${OUTDEV} conv=notrunc bs=512 seek=${START_ROOTFS_A}
 
 # Clean up temporary files.
 if [[ -n "${MBR_IMG:-}" ]]; then
   rm "${MBR_IMG}"
 fi