lib: membuff: fix readline not returning line in case of overflow

If line overflows readline it will not be returned, fix this behavior,
make it optional and documented properly.

Signed-off-by: Ion Agorria <ion@agorria.com>
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Link: https://lore.kernel.org/r/20240105072212.6615-6-clamor95@gmail.com


Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>