buildman: Include symbols in the read-only data section

When symbols switch between the inited data section and the read-only
data section their visbility changes, at present, with the -B option.

This is confusing, since adding 'const' to a variable declaration can
make it look like a significant improvement in bloat. But in fact
nothing has changed.

Add 'r' to the list of symbols types that are recorded, to correct this
problem. Add a constant to make it easier to find this code next time.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reported-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Tom Rini <trini@konsulko.com>