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/mapper.s

@@ -0,0 +1,32 @@
+.include "io.inc65"
+
+.importzp sp, sreg
+
+.export _mapper_enable
+.export _mapper_read, _mapper_write
+
+.autoimport on
+
+.code
+
+
+; void mapper_enable(uint8_t en)
+_mapper_enable:
+	sta MM_CTRL
+	rts
+
+_mapper_read:
+	phx
+	tax
+	lda MM_DATA,x
+	ldx #$00
+	rts
+
+_mapper_write:
+	phx
+	tax
+	jsr popa
+	sta MM_DATA,x
+	plx
+	rts
+