Added uart submodule

.gitmodules

@@ -1,3 +1,6 @@
 [submodule "io"]
 	path = io
 	url = git@git.blackmark.me:avr/io.git
+[submodule "uart"]
+	path = uart
+	url = git@git.blackmark.me:avr/uart.git

README.md

@@ -52,12 +52,12 @@ The PWM mode of the hardware timer can also be used. Additionally, if PWM is nee
     - [ ] Support all hardware provided configuration options
     - [ ] Compile-time check for correct hardware configuration
 - UART
-    - [ ] Separation of interface and backend-driver
+    - [x] Separation of interface and backend-driver
     - [ ] Support all hardware provided configuration options
     - [ ] Compile-time check for correct hardware configuration
     - [ ] Software backend driver
-    - [ ] Blocking hardware backend
-    - [ ] Interrupt driven hardware backend
+    - [x] Blocking hardware backend
+    - [x] Interrupt driven hardware backend
     - [ ] Type-safe convenience functions for writing and reading basic data types
 - SPI
     - [ ] Separation of interface and backend-driver