Remove autogenerated files from .gitignore file

Keep them in version control from now on.

src/.gitignore

@@ -1,2 +0,0 @@
-# Generated automatically.
-config.rs

src/config.rs

@@ -0,0 +1,2 @@
+/// The clock frequency of device being targeted in Hertz.
+pub const CPU_FREQUENCY_HZ: u32 = 16000000;

src/cores/.gitignore

@@ -1 +0,0 @@
-*.rs

src/cores/mod.rs

@@ -0,0 +1,6 @@
+
+/// The ATmega328.
+pub mod atmega328;
+#[cfg(avr_mcu_atmega328)]
+pub use self::atmega328 as current;
+