Replace bios with bootloader, don't use bootsector

Instead of using the bootsector to load a bootloader, just put the
bootloader in the ROM and call it a day. It looks for a file on the SD
card in the root directory named `kernel.bin` and loads it into memory.

This is not a perfect solution, as the kernel will grow larger and the
kernel load address will have to change. At this point I could add back
the bootloader, but that is for later.

Also the bootloader is just a copy of the kernel, so that can be trimmed
down a lot.

sw/bootloader/irq.c

@@ -0,0 +1,24 @@
+
+#include <stdint.h>
+#include <conio.h>
+
+#include "devices/interrupt.h"
+#include "devices/uart.h"
+
+char lastchar;
+
+
+void handle_irq() {
+    uint8_t status;
+
+    status = irq_get_status();
+
+    if (status & BUTTON) {
+        cputs("Button Interrupt!\n");
+        irq_set_status(status & ~BUTTON);
+    }
+    if (status & UART) {
+        lastchar = uart_rxb();
+        irq_set_status(status & ~UART);
+    }
+}