Reland #2: "Split the cros_boards gclient var into two for fetching QEMU images."

This is the 2nd reland of 8f372d5fc82776cbb0a147e46dbfbfd073df411e

Reason for reland:
 - Fixed yet-another DEPS-parser in crbug.com/1106435
 - No changes in this CL.

Original change's description:
> Split the cros_boards gclient var into two for fetching QEMU images.
>
> This will help with the TODO at:
> https://source.chromium.org/chromium/chromium/src/+/master:build/config/chromeos/rules.gni;drc=ded9eb5153bb13b3f8821a6fa6fac6b3f0f590fd;l=22
>
> It will also remove the need for hard-coding the names of all these
> QEMU-compatible boards.
>
> Bug: 937821
> Change-Id: Iab180fc012bd694b4908ba0654da93d42e2274fa
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2265498
> Commit-Queue: Ben Pastene <bpastene@chromium.org>
> Reviewed-by: Erik Chen <erikchen@chromium.org>
> Reviewed-by: Dirk Pranke <dpranke@google.com>
> Cr-Commit-Position: refs/heads/master@{#786005}

Bug: 937821, 1106435
Change-Id: Ia1d47740c4061739f4ad4dfe403f755fd68fdc4d
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2303377
Reviewed-by: Erik Chen <erikchen@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@google.com>
Commit-Queue: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#789293}
GitOrigin-RevId: 772fb497a240842e752967fb6e878b00f9fe2c2d

args/chromeos/README.md

@@ -3,9 +3,12 @@ This directory is used to store GN arg mapping for Chrome OS boards.
 Files in this directory are populated by running `gclient sync` with specific
 arguments set in the .gclient file. Specifically:
 * The file must have a top-level variable set: `target_os = ["chromeos"]`
-* The `"custom_vars"` parameter of the chromium/src.git solution must include the
+* The `"custom_vars"` parameter of the chromium/src.git solution must include
-  parameter: `"cros_boards": "{BOARD_NAMES}"` where `{BOARD_NAMES}` is a
+  the parameter: `"cros_boards": "{BOARD_NAMES}"` where `{BOARD_NAMES}` is a
   colon-separated list of boards you'd like to checkout.
+* If you'd like to a checkout a QEMU-bootable image for a given board, include
+  it in the `cros_boards_with_qemu_images` var rather than the `cros_boards`
+  var.
 
 A typical .gclient file is a sibling of the src/ directory, and might look like
 this:
@@ -18,7 +21,10 @@ solutions = [
     "custom_deps": {},
     "custom_vars" : {
         "checkout_src_internal": True,
-        "cros_boards": "eve",
+        "cros_boards": "eve:kevin",
+        # If a QEMU-bootable image is desired for any board, move it from
+        # the previous var to the following:
+        "cros_boards_with_qemu_images": "amd64-generic",
     },
   },
 ]