arm: working bootloader, example app

'make' will produce two outputs:
- bootloader.elf to be loaded into the bitstream
- app.bin to be loaded through the programmer

Loading app.bin is as simple as:
- reset the board
- `python3 prog.py app.bin`

arm/uart.h

@@ -0,0 +1,7 @@
+#pragma once
+
+#include "xuartlite.h"
+
+extern XUartLite* uart0;
+
+void InitUarts();