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>

include/fastboot-internal.h

@@ -18,6 +18,13 @@ extern u32 fastboot_buf_size;
  */
 extern void (*fastboot_progress_callback)(const char *msg);
 
+/**
+ * fastboot_getvar_all() - Writes current variable being listed from "all" to response.
+ *
+ * @response: Pointer to fastboot response buffer
+ */
+void fastboot_getvar_all(char *response);
+
 /**
  * fastboot_getvar() - Writes variable indicated by cmd_parameter to response.
  *