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/tests/test_main.c

@@ -0,0 +1,89 @@
+#include <stdio.h>
+
+#include "devices/board_io.h"
+#include "devices/uart.h"
+#include "devices/interrupt.h"
+
+int main(void)
+{
+    int retval = 0;
+
+    int i;
+
+    printf("\nStarting tests...\n\n");
+
+    printf("Testing hex_set_8...\n");
+    for (i = 0; i < 3; i++) {
+        if (hex_set_8(i+1, i)) {
+            printf("Failed to write to idx %d!\n", i);
+            retval++;
+        }
+        if (*(uint8_t*)0x7ff0+i != i+1) {
+            printf("Incorrect value at idx %d!\n", i);
+            retval++;
+        }
+    }
+
+    if (!hex_set_8(0xab, 3)) {
+        printf("Writing to idx 3 should fail!\n");
+        retval++;
+    }
+    printf("Done!\n\n");
+
+    printf("Testing hex_set_16...\n");
+    if (hex_set_16(0xabcd)){
+        printf("Failed to write!\n");
+    }
+    if (*(uint16_t*)0x7ff0 != 0xabcd) {
+        printf("Incorrect value!\n", i);
+        retval++;
+    }
+    printf("Done!\n\n");
+
+    printf("Testing hex_set_24...\n");
+    if (hex_set_24(0xabcdef)){
+        printf("Failed to write!\n");
+    }
+    if (*(uint16_t*)0x7ff0 != 0xcdef && *(uint8_t*)0x7ff2 != 0xab) {
+        printf("Incorrect value!\n", i);
+        retval++;
+    }
+    printf("Done!\n\n");
+
+    printf("Testing hex_enable...\n");
+    hex_enable(0xa5);
+    if (*(uint8_t*)0x7ff3 != 0xa5) {
+        printf("Incorrect value!\n", i);
+        retval++;
+    }
+    printf("Done!\n\n");
+
+    printf("Testing uart_txb_block...\n");
+    *(uint8_t*)0x7ff5 = 0;
+    uart_txb_block(0xa5);
+    if (*(uint8_t*)0x7ff4 != 0xa5) {
+        printf("Incorrect value!\n", i);
+        retval++;
+    }
+    printf("Done!\n\n");
+
+    printf("Testing uart_status...\n");
+    *(uint8_t*)0x7ff5 = 0xa5;
+    if (uart_status() != 0xa5) {
+        printf("Incorrect value!\n", i);
+        retval++;
+    }
+    printf("Done!\n\n");
+
+
+    printf("Testing irq_get_status...\n");
+    *(uint8_t*)0x7fff = 0xa5;
+    if (irq_get_status() != 0xa5) {
+        printf("Incorrect value!\n", i);
+        retval++;
+    }
+    printf("Done!\n\n");
+
+
+    return retval != 0;
+}