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/gpio.h

@@ -0,0 +1,9 @@
+#pragma once
+
+#include <cstdint>
+
+struct Gpio {
+    volatile uint32_t data;
+};
+
+#define gpio0 ((Gpio*) 0x40000000)