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.cc

@@ -0,0 +1,21 @@
+#include "uart.h"
+
+namespace {
+
+constexpr uintptr_t kUart0BaseAddress = 0x40001000;
+XUartLite uart0_inst;
+XUartLite_Config uart0_config = {
+    .DeviceId = 0,
+    .RegBaseAddr = kUart0BaseAddress,
+    .BaudRate = 115200,
+    .UseParity = false,
+    .DataBits = 8,
+};
+
+}  // namespace
+
+XUartLite* uart0 = &uart0_inst;
+
+void InitUarts() {
+    XUartLite_CfgInitialize(uart0, &uart0_config, uart0_config.RegBaseAddr);
+}