Loading from disk works, but jumping to the kernel does not

Makefile

@@ -1,16 +1,21 @@
 all: boot.img kernel.bin
 
-boot.bin: asm/bootloader.S asm/bootloader.S
-	nasm asm/bootloader.S -f bin -o $@
-
-asm/%.o: asm/%.S
-	nasm $< -f as86 -o $@
-
 src/%.o: src/%.c
 	bcc -ansi -3 -c -o $@ $<
 
 kernel.bin: src/kernel.o
-	ld86 -T0x1000 -o $@ $^
+	ld86 -T0x1000 -M -o $@ $^
+
+asm/kernel_syms.S: kernel.bin
+	objdump86 kernel.bin | \
+		grep -E "^[0-9]+ T _.*" | \
+		python -c "import sys; print '\n'.join([\"_extern_c%s:\n    jmp 0x1000:0x%04x\" % (x.split(' ')[2].strip('\n'), int(x.split(' ')[0].lstrip('0'), 16)-0x1000) for x in sys.stdin.readlines()])" > asm/kernel_syms.S
+
+boot.bin: asm/kernel_syms.S asm/bootloader.S asm/bootloader.S
+	cd asm && nasm bootloader.S -f bin -o ../$@
+
+asm/%.o: asm/%.S
+	nasm $< -f as86 -o $@
 
 boot.img: boot.bin kernel.bin
 	cat $^ > $@