build: Go back to the "legacy" identifier theme of svd2rust

After starting work on porting avr-hal to the new version of avr-device,
I started hitting the effects of the changed identifiers in the
generated API.  In Rahix/avr-hal#545, commit Rahix/avr-hal@ef69e1b, I at
least got to the point of getting the Arduino Uno examples to compile
again.

I have to say, I absolutely do not like the code churn we introduce with
this.  While it is probably manageable for avr-hal with team effort,
that's not all:

 - All downstream users will have to work through the same churn of
   (sometimes non-trivial) identifier changes.
 - All current PRs in avr-hal will need to be rebased by their authors
   to the new style identifiers.
 - Legacy examples found on the web all become outdated.
 - Rust compiler error messages on fixing the names are quite
   hit-or-miss.  The update strategy is not clear if you don't
   understand what has happened.

I took a look around the ecosystem and others have also decided to stick
to what svd2rust calls the "legacy" identifier theme.  I am much in
favor of doing the same.  There is no significant advantage to doing
this change now, so let's not waste huge amounts of effort on it.

Author: Rahix

build.rs

@@ -266,7 +266,7 @@ impl CodeGenerator {
         svd2rust_config.skip_crate_attributes = true;
         svd2rust_config.reexport_core_peripherals = false;
         svd2rust_config.reexport_interrupt = false;
-        svd2rust_config.ident_formats = IdentFormats::default_theme();
+        svd2rust_config.ident_formats = IdentFormats::legacy_theme();
 
         let generated_stream =
             svd2rust::generate::device::render(&svd, &svd2rust_config, &mut String::new())