spl: Add a size check for TPL

spl: Add a size check for TPL

We have the ability to enforce a maximum size for SPL but not yet for TPL.
Add a new option for this.

Document the size check macro while we are here.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>