bmpblk: Add screenshot board config

Add a "screenshot" entry to boards.yaml, which will be used by
depthcharge's `make screenshot` to generate the bitmaps at build time,
instead of checking in prebuilt bitmaps. Use the default DPI, so that
the text bitmaps are rendered in high resolution and then downscaled by
depthcharge, making the screenshots look sharper.

BUG=b:551743258
BRANCH=none
TEST=LOCALES=en BOARD=screenshot make
TAG=agy
CONV=4d411a1c-369f-4fd3-ac8a-7bd726afa3b5

Change-Id: I5f73993e4dff883e340818fac6650bee6a6a6964
Signed-off-by: Yu-Ping Wu <yupingso@google.com>
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/bmpblk/+/8459506
Tested-by: Yu-Ping Wu <yupingso@chromium.org>
Reviewed-by: Chen-Tsung Hsieh <chentsung@chromium.org>
Commit-Queue: Yu-Ping Wu <yupingso@chromium.org>
GitOrigin-RevId: 465b70c9059737ed6ea9a86a5c504ed0d520deb6