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/io.inc65

@@ -0,0 +1,18 @@
+SEVEN_SEG   = $7ff0
+
+UART        = $7ff4
+UART_TXB    = UART
+UART_RXB    = UART
+UART_STATUS = UART + 1
+
+LED         = $7ff6
+SW          = LED
+
+MM_CTRL		= $7ff7
+MM_DATA		= $7fe0
+
+SD_ARG      = $7ff8
+SD_CMD      = $7ffc
+SD_DATA     = $7ffd
+
+IRQ_STATUS  = $7fff