Introduce rbutil/mkzenboot, a replacement for tools/mkzenboot

The old tools/mkzenboot has a number of problems: very hard to maintain,
poor integration with rbutil. Restart from scratch by recycling all the
crypto and descrambling code, rewrite the actual firmware modification part
to handle all scenarios in a much clearer way. The code is ready to be
integrated into Rockbox Utility, by using the very similar interface to
mkimxboot.
I copied all the keys from the old mkzenboot, so it can potentially support
the older Creative ports, but since this is untested, I prefer not do so
at the moment. However, I did add a "mixed" boot option to support the
dualboot style used in the older ports.

Change-Id: I80cfc48fa78187baa1b1692e8a30ec7137cea37b

rbutil/mkzenboot/Makefile

@@ -0,0 +1,30 @@
+#             __________               __   ___.
+#   Open      \______   \ ____   ____ |  | _\_ |__   _______  ___
+#   Source     |       _//  _ \_/ ___\|  |/ /| __ \ /  _ \  \/  /
+#   Jukebox    |    |   (  <_> )  \___|    < | \_\ (  <_> > <  <
+#   Firmware   |____|_  /\____/ \___  >__|_ \|___  /\____/__/\_ \
+#                     \/            \/     \/    \/            \/
+
+# We use the HMAC-SHA1 code from tools/
+TOOLS_DIR=../../tools/
+CFLAGS += -Wall -I$(TOOLS_DIR)
+CFLAGS += -std=c99 -g
+LDOPTS += -lz
+
+OUTPUT = mkzenboot
+
+# inputs for lib
+TOOLS_SOURCES = hmac-sha1.c
+LIBSOURCES := dualboot.c mkzenboot.c md5.c utils.c \
+	      $(addprefix $(TOOLS_DIR),$(TOOLS_SOURCES))
+
+# inputs for binary only
+SOURCES := $(LIBSOURCES) main.c
+# dependencies for binary
+EXTRADEPS :=
+
+include ../libtools.make
+
+# explicit dependencies on dualboot.{c,h} and mkimxboot.h
+$(OBJDIR)mkzenboot.o: dualboot.h dualboot.c mkzenboot.c mkzenboot.h
+$(OBJDIR)main.o: dualboot.h dualboot.c main.c mkzenboot.h