common: avb_verify: add str_avb_io_error/str_avb_slot_error

Introduce str_avb_io_error() and str_avb_slot_error() functions,
that provide a pointer to AVB runtime error message.

Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Link: https://lore.kernel.org/r/20240209192045.3961832-6-igor.opaniuk@foundries.io


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