Write the image to the correct target app partition

Perhaps I am misunderstanding what the --target-app-partition flag is
supposed to do, but the code previously always wrote the image to the
"factory" app partition if one was present, regardless of the partition
specified with the `--target-app-partition` flag.

This change updates the code to write the image to the partition
specified with the `--target-app-partition` flag.

I confirmed that using `espflash flash --target-app-partition ota_0` now
causes espflash to write the image to the `ota_0` partition.  Note that
this leaves the `factory` partition and any `ota_data` partition
unchanged, so the bootloader will still boot from the factory partition
by default, but this makes it possible to write new app images to the
OTA partitions for testing purposes.

espflash/src/image_format.rs

@@ -259,7 +259,7 @@ impl<'a> IdfBootloaderFormat<'a> {
         let app_size = data.len() as u32;
         let part_size = target_app_partition.size();
 
-        // The size of the application must not exceed the size of the factory
+        // The size of the application must not exceed the size of the target app
         // partition.
         if app_size as f32 / part_size as f32 > 1.0 {
             return Err(Error::ElfTooBig(app_size, part_size));
@@ -307,22 +307,8 @@ impl<'a> IdfBootloaderFormat<'a> {
             data: Cow::Owned(self.partition_table.to_bin().unwrap()),
         };
 
-        let app_partition = self
-            .partition_table
-            .find("factory")
-            .or_else(|| self.partition_table.find_by_type(Type::App))
-            .expect("no application partition found");
-
-        if self.flash_segment.data.len() > app_partition.size() as usize {
-            panic!(
-                "image size ({} bytes) is larger partition size ({} bytes)",
-                self.flash_segment.data.len(),
-                app_partition.size()
-            );
-        }
-
         let app_segment = RomSegment {
-            addr: app_partition.offset(),
+            addr: self.flash_segment.addr,
             data: Cow::Borrowed(&self.flash_segment.data),
         };