video: theadorable: Use RGB565 for BMP blitting

At present this uses RGB555 format for blitting to a display. Sandbox uses
565 and that seems to be more normal for BMP as well. Update the code
accordingly and add a test.

Note that this likely breaks the theadorable board so we may need to
discuss supporting both formats.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/configs/sandbox_defconfig b/configs/sandbox_defconfig
index 5dffc70..4f41358 100644
--- a/configs/sandbox_defconfig
+++ b/configs/sandbox_defconfig
@@ -286,6 +286,7 @@
 CONFIG_SPLASH_SCREEN_ALIGN=y
 CONFIG_VIDEO_BMP_RLE8=y
 CONFIG_BMP_16BPP=y
+CONFIG_BMP_24BPP=y
 CONFIG_W1=y
 CONFIG_W1_GPIO=y
 CONFIG_W1_EEPROM=y