config_distro_bootcmd.h: add note on error handling

This should make it more clear why there appear to be C pre-processor
symbols in the file that contain mixed case. They're really error
messages.

Suggested-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Simon Glass <sjg@chromium.org>