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/Makefile

@@ -1,14 +1,11 @@
-.PHONY: all install bios bootloader kernel clean
+.PHONY: all install bootloader kernel clean
 
-all: bios bootloader kernel
+all: bootloader kernel
 
 install: all
 	sh script/format_disk.sh
 	sh script/copy_files.sh
 
-bios:
-	@$(MAKE) -C bios
-
 bootloader:
 	@$(MAKE) -C bootloader
 
@@ -17,6 +14,5 @@ kernel:
 
 
 clean:
-	@$(MAKE) -C bios  --no-print-directory $@
 	@$(MAKE) -C bootloader  --no-print-directory $@
 	@$(MAKE) -C kernel  --no-print-directory $@