Fix elf build; rename target file rom.bin to bios.bin.

The main output file is now called out/bios.bin (instead of out/rom.bin).
Use ld to build final elf file and call the result out/bios.bin.elf
Make sure to long jump from external 32bit entry point.
diff --git a/Makefile b/Makefile
index 3d7c3e2..c8e68bd 100644
--- a/Makefile
+++ b/Makefile
@@ -30,7 +30,7 @@
 TABLETMP=$(addprefix $(OUT), $(patsubst %.c,%.16.s,$(TABLESRC)))
-all: $(OUT) $(OUT)rom.bin $(TABLETMP)
+all: $(OUT) $(OUT)bios.bin $(TABLETMP)
 # Run with "make V=1" to see the actual compile commands
 ifdef V
@@ -103,7 +103,7 @@
 	@echo "  Linking $@"
 	$(Q)ld -T $(OUT) $< -o $@
-$(OUT)rom.bin: $(OUT)rom16.bin $(OUT)rom32.bin $(OUT) $(OUT)
+$(OUT)bios.bin: $(OUT)rom16.bin $(OUT)rom32.bin $(OUT) $(OUT)
 	@echo "  Building $@"