Write the image to the correct target app partition

This updates the code to honor the --target-app-partition flag,
and write the application image to the specified partition.  Previously
the code always wrote the image to the "factory" app partition if one
was present, regardless of the partition specified with the
`--target-app-partition` flag.  (It seems like this behavior was likely
broken in d886d33, which updated the code to always write to the
factory partition again.)

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.

CHANGELOG.md

@@ -12,6 +12,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ### Fixed
 - Fixed help text for size parameter of read-flash subcommand
+- Fixed behavior of the --target-app-partition flag (#634)
 
 ### Changed
 

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),
         };