Add preliminary bootloader

This bootloader right now just prints the letter A, but should be
capable of reading blocks from the SD card into memory.

sw/bootloader/main.c

@@ -0,0 +1,11 @@
+#include <stdint.h>
+
+#include "bios_calls.h"
+#include "uart.h"
+
+//Should probably do this in asm
+void load_bootsect() {
+    uart_txb_block('A');
+    for (;;);
+    return;
+}