AK3: resolve non-zero return code introduced by `cat <img> /dev/zero …

…> <block>`

- the expected "cat: write error: No space left on device" gives a non-zero return code so always return true
- devices that flash dtbo abort since the return code would get passed back to the Backend and silently fail as the final command in anykernel.sh (via `write_boot`), so catch seperately with a proper error message for other image write methods

tools/ak3-core.sh

@@ -321,7 +321,10 @@ flash_boot() {
     dd if=/dev/zero of=$block $customdd 2>/dev/null;
     dd if=boot-new.img of=$block $customdd;
   else
-    cat boot-new.img /dev/zero > $block 2>/dev/null;
+    cat boot-new.img /dev/zero > $block 2>/dev/null || true;
+  fi;
+  if [ $? != 0 ]; then
+    abort "Flashing image failed. Aborting...";
   fi;
 }
 
@@ -349,7 +352,10 @@ flash_dtbo() {
       dd if=/dev/zero of=$dtboblock 2>/dev/null;
       dd if=$dtbo of=$dtboblock;
     else
-      cat $dtbo /dev/zero > $dtboblock 2>/dev/null;
+      cat $dtbo /dev/zero > $dtboblock 2>/dev/null || true;
+    fi;
+    if [ $? != 0 ]; then
+      abort "Flashing dtbo failed. Aborting...";
     fi;
   fi;
 }