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/conio.s

@@ -0,0 +1,60 @@
+.importzp sp, sreg
+
+.import _uart_txb_block
+.import _lastchar
+
+.export _cputc
+.export gotoxy
+.export _clrscr
+.export _cgetc
+
+.autoimport	on
+
+.code
+
+; void __fastcall__ cputc (char c);
+_cputc:
+        jsr _uart_txb_block
+        cmp #$0a
+        bne @1
+        lda #$0d
+        jsr _uart_txb_block
+@1:     rts
+
+; void __fastcall__ gotoxy (unsigned char x, unsigned char y);
+gotoxy:
+        phx
+        phy
+        tay                         ; Move y position to y
+        lda (sp)
+        tax                         ; Move x position to x
+        lda #$1b
+        jsr _uart_txb_block
+        lda #'['
+        jsr _uart_txb_block
+        tya
+        jsr _uart_txb_block
+        lda #';'
+        jsr _uart_txb_block
+        txa
+        jsr _uart_txb_block
+        lda #'H'
+        jsr _uart_txb_block
+        ply
+        plx
+        rts
+
+_clrscr:
+        phx
+        lda #$1b
+        jsr _uart_txb_block
+        lda #'c'
+        jsr _uart_txb_block
+        pla
+        rts
+
+_cgetc:
+@2:     lda _lastchar
+        beq @2
+        stz _lastchar
+        rts