raspberry-pi/4/dtmerge: fix application of overlays

Previously whenever an overlay was found to be incompatible with a base
device tree blob, the entire base dtb would be skipped in favor of
processing the next one. This had the unfortunate effect where overlays
would not fully be applied if any incompatibility was found. For
example, this is an issue with build device trees specific for one
flavor of raspberry pi if the overlay was not compatible _everywhere_.

The solution is to forego the `continue` keyword if an overlay is in
compatible and instead use a compound conditional statement to skip
incompatible overlays but continue trying to apply it to any remaining
dtbs.

raspberry-pi/4/apply-overlays-dtmerge.nix

@@ -24,15 +24,12 @@ with lib; (base: overlays': stdenvNoCC.mkDerivation {
       # skip incompatible and non-matching overlays
       if [[ ! "$dtbCompat" =~ "$overlayCompat" ]]; then
         echo "Skipping overlay ${o.name}: incompatible with $(basename "$dtb")"
-        continue
-      fi
-      ${optionalString ((o.filter or null) != null) ''
-      if [[ "''${dtb//${o.filter}/}" ==  "$dtb" ]]; then
-        echo "Skipping overlay ${o.name}: filter does not match $(basename "$dtb")"
-        continue
-      fi
+      elif ${if ((o.filter or null) == null) then "false" else ''
+        [[ "''${dtb//${o.filter}/}" ==  "$dtb" ]]
       ''}
-
+      then
+        echo "Skipping overlay ${o.name}: filter does not match $(basename "$dtb")"
+      else
         echo -n "Applying overlay ${o.name} to $(basename "$dtb")... "
         mv "$dtb"{,.in}
 
@@ -44,6 +41,7 @@ with lib; (base: overlays': stdenvNoCC.mkDerivation {
 
         echo "ok"
         rm "$dtb.in" "$dtboWithExt"
+      fi
       '')}
 
     done'';