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_flattree_defconfig b/configs/sandbox_flattree_defconfig
index f16d890..4d5a73f 100644
--- a/configs/sandbox_flattree_defconfig
+++ b/configs/sandbox_flattree_defconfig
@@ -199,6 +199,7 @@
 CONFIG_SANDBOX_OSD=y
 CONFIG_VIDEO_BMP_RLE8=y
 CONFIG_BMP_16BPP=y
+CONFIG_BMP_24BPP=y
 CONFIG_CMD_DHRYSTONE=y
 CONFIG_RSA_VERIFY_WITH_PKEY=y
 CONFIG_TPM=y