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/board_io.s

@@ -0,0 +1,71 @@
+.include "io.inc65"
+
+.importzp sp, sreg
+
+.export _hex_set_8
+.export _hex_set_16
+.export _hex_set_24
+.export _hex_enable
+.export _sw_read
+.export _led_set
+
+.autoimport	on
+
+.code
+
+; @in A: idx Stack[0]: val
+; @out A: 0 for success, 1 for failure.
+; Sets one of the 3 pairs of hex digits.
+_hex_set_8:
+        phx
+        cmp #$3         ; If idx >= 3 then fail
+        bcc @1
+        plx
+        lda #$1
+        rts
+@1:     tax             ; Move idx into x
+        jsr popa        ; put val into a
+        sta SEVEN_SEG,x ; write to val
+        lda #$0
+        plx
+        rts
+
+; @in A/X: val
+; @out A: 0 for success, 1 for failure
+; Sets the low 2 pairs of hex digits
+_hex_set_16:
+        sta SEVEN_SEG
+        stx SEVEN_SEG+1
+        lda #$0
+        rts
+
+; @in A/X/sreg: val
+; @out A: 0 for success, 1 for failure
+; Sets the 3 pairs of hex digits for a 24 bit value
+_hex_set_24:
+        sta SEVEN_SEG
+        stx SEVEN_SEG+1
+        lda sreg
+        sta SEVEN_SEG+2
+        lda #$0
+        rts
+
+; @in A: mask
+; Set the mask for seven seg enables
+_hex_enable:
+        sta SEVEN_SEG+3
+        rts
+
+; @out A: The Value of the switches
+; Reads the current values of the switches.
+_sw_read:
+        lda SW
+        ldx #$0
+        rts
+
+; @in A: val
+; @out A: 0 for success, 1 for failure
+; Sets the LEDs
+_led_set:
+        sta LED
+        rts