[mbr] "Press any key to boot from USB..."

* Adds an MBR to prompt the use about USB or HDD boot
* If USB boot is not selected, an INT13h override is added to
  make the second bootable drive think it is first (0x81 -> 0x80)
* USB drive BIOS index is left unmodified

res/mbr/Makefile

@@ -0,0 +1,60 @@
+#
+# This file is part of the Rufus project.
+#
+# Copyright (c) 2012 Pete Batard <pete@akeo.ie>
+#
+# This program is free software; you can redistribute it and/or
+# modify it under the terms of the GNU General Public License as
+# published by the Free Software Foundation, either version 3 of
+# the License, or (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program; if not, see <http://www.gnu.org/licenses/>.
+#
+
+OBJECTS   = mbr.o
+TARGET    = mbr
+
+ASM       = gcc
+CC        = gcc
+LD        = ld
+OBJDUMP   = objdump
+OBJCOPY   = objcopy
+CFLAGS    = -m32
+LDFLAGS   = -nostartfile
+
+.PHONY: all clean
+
+all: $(TARGET).bin
+
+clean:
+	@-rm -f -v *.o *.out $(TARGET).map
+
+%.o: %.c Makefile
+	@echo "[CC]  $@"
+	@$(CC) -c -o $*.o $(CFLAGS) $<
+
+%.o: %.S Makefile
+	@echo "[AS]  $<"
+	@$(ASM) -c -o $*.o $(CFLAGS) $<
+
+# Produce a disassembly dump, for verification purposes
+dis: $(TARGET).out
+	@echo "[DIS] $<"
+	@$(OBJCOPY) -O binary -j .main --set-section-flags .main=alloc,load,readonly,code $< main.bin
+	@$(OBJDUMP) -D -bbinary -mi8086 -Mintel main.bin | less
+	@-rm -f main.bin
+
+$(TARGET).out: $(OBJECTS) $(TARGET).ld
+	@echo "[LD]  $@"
+	@$(LD) $(LDFLAGS) -T$(TARGET).ld -o $@ $(OBJECTS) -Map $(TARGET).map
+
+$(TARGET).bin: $(TARGET).out
+	@echo "[MBR] $@"
+	@# Note: -j only works for sections that have the 'ALLOC' flag set
+	@$(OBJCOPY) -O binary -j .main --gap-fill=0x00 $< $@