Patch by dnivra (GSoC 2014, ARM suppport)

* Cleanup the SD card image building to allow jam -q @bootstrap-mmc to
work.

There are a few remaining tricks before you can safely build an image:
* This uses a non-POSIX du option, and is only tested with Linux du
only (Linux is the only supported system to run bootstrap builds,
anyway)
* The Python recipe in haikuports.cross is known to not build on
Debian/Ubuntu, but work fine on OpenSuse. There is a patch available in
haikuports bugtracker to allow the reverse.
* You need to populate the haikuports repo package list with some
packages (which don't exist yet) to make the build system happy. But our
git hook to generate the repositories is preventnig me to share this
hack.

Once built, the image currently crashes early in the kernel execution.
On to debug that!

build/jam/BuildSetup

@@ -84,6 +84,11 @@ HAIKU_DEFAULT_ANYBOOT_NAME = haiku-anyboot.image ;
 HAIKU_DEFAULT_ANYBOOT_DIR = $(HAIKU_OUTPUT_DIR) ;
 HAIKU_DEFAULT_ANYBOOT_LABEL ?= Haiku ;
 
+# Haiku MMC defaults
+HAIKU_DEFAULT_MMC_IMAGE_NAME = haiku-mmc.image ;
+HAIKU_DEFAULT_MMC_DIR = $(HAIKU_OUTPUT_DIR) ;
+HAIKU_DEFAULT_MMC_LABEL ?= Haiku ;
+
 # analyze and optionally replace jam's target parameters
 ProcessCommandLineArguments ;