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/devices/uart.h

@@ -0,0 +1,13 @@
+#ifndef _UART_H
+#define _UART_H
+
+#include <stdint.h>
+
+void uart_txb(uint8_t val);
+void uart_txb_block(uint8_t val);
+
+uint8_t uart_rxb();
+
+uint8_t uart_status();
+
+#endif