fastboot: implement "getvar all"

This commit implements "fastboot getvar all" listing
by iterating the existing dispatchers that don't require
parameters (as we pass NULL), uses fastboot multiresponse.

Signed-off-by: Ion Agorria <ion@agorria.com>
Signed-off-by: Svyatoslav Ryhel <clamor95@gmail.com>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20240105072212.6615-3-clamor95@gmail.com


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