[coreboot-gerrit] Patch merged into coreboot/master: 50a8174 libpayload: Add remote GDB support

the following patch was just integrated into master:
commit 50a81748fd8db14ad1da5bdb9d43788f1c4173e1
Author: Julius Werner 
Date:   Thu May 15 11:57:38 2014 -0700

libpayload: Add remote GDB support

This patch adds the ability to attach a GDB host through the UART to a
    running payload. Libpayload implements a small stub that can parse and
    respond to the GDB remote protocol and provide the required primitives
    (reading/writing registers/memory, etc.) to allow GDB to control
    execution.

The goal of this implementation is to be as small and uninvasive as
    possible. It implements only the minimum amount of primitives required,
    and relies on GDB's impressive workaround capabilities (such as
    emulating breakpoints by temporarily replacing instructions) for the
    more complicated features. This way, a relatively tiny amount of code on
    the firmware side opens a vast range of capabilities to the user, not
    just in debugging but also in remote-controlling the firmware to change
    its behavior (e.g. through GDBs ability to modify variables and call
    functions).

By default, a system with the REMOTEGDB Kconfig will only trap into GDB
    when executing halt() (including the calls from die_if(), assert(), and
    exception handlers). In addition, payloads can manually call gdb_enter()
    if desired. It will print a final "Ready for GDB connection." on the
    serial, detach the normal serial output driver and wait for the commands
    that GDB starts sending on attach.

Based on original implementation by Gabe Black .

BUG=chrome-os-partner:18390
    TEST=Boot a GDB enabled image in recovery mode (or get it to hit a
    halt()), close your terminal, execute '<toolchain>-gdb --symbols
    /build/<board>/firmware/depthcharge_gdb/depthcharge.elf --directory
    ~/trunk/src/third_party/coreboot/payloads/libpayload --directory
    ~/trunk/src/platform/depthcharge --directory
    ~/trunk/src/platform/vboot_reference --ex "target remote
    <cpu_uart_pty>"' and behold the magic.
    (You can also SIGSTOP your terminal's parent shell and the terminal
    itself, and SIGCONT them in reverse order after GDB exits. More
    convenient wrapper tools to do all this automatically coming soon.)

Original-Change-Id: Ib440d1804126cdfdac4a8801f5015b4487e25269
    Original-Signed-off-by: Julius Werner 
    Original-Reviewed-on: https://chromium-review.googlesource.com/202563
    Original-Reviewed-by: Stefan Reinauer 
    (cherry picked from commit 9c4a642c7be2faf122fef39bdfaddd64aec68b77)
    Signed-off-by: Marc Jones

Change-Id: I9238b4eb19d3ab2c98e4e1c5946cd7d252ca3c3b
    Reviewed-on: http://review.coreboot.org/8119
    Tested-by: build bot (Jenkins)
    Reviewed-by: Stefan Reinauer

See http://review.coreboot.org/8119 for details.

-gerrit