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.s

@@ -0,0 +1,36 @@
+.include "io.inc65"
+
+.importzp sp, sreg
+
+.export _uart_txb, _uart_txb_block
+.export _uart_rxb
+.export _uart_status
+
+.autoimport	on
+
+.code
+
+; @in A: byte to transmit
+; Transmits a byte over the UART
+_uart_txb:
+        sta UART_TXB        ; Just write value, don't wait
+        rts
+
+_uart_txb_block:
+        pha
+        sta UART_TXB        ; Write value
+@1:     lda UART_STATUS     ; Wait for status[0] to be 0
+        bit #$01
+        bne @1
+        pla
+        rts
+
+_uart_rxb:
+        lda UART_RXB        ; Read value
+        ldx #$00
+        rts
+
+_uart_status:
+        lda UART_STATUS
+        ldx #$00
+        rts