docs: Add a payload section about coreboot support #116

Signed-off-by: Shawn Chang <citypw@gmail.com>

diff --git a/docs/external/coreboot.md b/docs/external/coreboot.md
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+OpenSBI as coreboot payload
+==============================
+
+[coreboot](https://www.coreboot.org/) is a free/libre and open source firmware platform support multiple hardware architectures( x86, ARMv7, arm64, PowerPC64, MIPS and RISC-V) and diverse hardware models. In RISC-V world, coreboot currently support HiFive Unleashed with OpenSBI as a payload to boot GNU/Linux:
+
+```
+SiFive HiFive unleashed's original firmware boot process:
+                                    +-----------+
++------+    +------+    +------+    | BBL       |
+| MSEL |--->| ZSBL |--->| FSBL |--->|   +-------+
++------+    +------+    +------+    |   | linux |
+                                    +---+-------+
+
+coreboot boot process:
+                      +---------------------------------------------------------------------+
+                      | coreboot                                                            |
++------+   +------+   |  +-----------+  +----------+  +----------+  +-----------------------+
+| MSEL |-->| ZSBL |-->|  | bootblock |->| romstage |->| ramstage |->| payload ( OpenSBI)    |
++------+   +------+   |  +-----------+  +----------+  +----------+  |             +-------+ |
+                      |                                             |             | linux | |
+                      +---------------------------------------------+-------------+-------+-+
+```
+
+The upstreaming work is still in progress. There's a [documentation](https://github.com/hardenedlinux/embedded-iot_profile/blob/master/docs/riscv/hifiveunleashed_coreboot_notes-en.md) about how to build [out-of-tree code](https://github.com/hardenedlinux/coreboot-HiFiveUnleashed) to load OpenSBI.