Switch to CyanogenMod busybox

Switch to CyanogenMod busybox, as the one by Freescale in repository
platform_external_busybox is missing an Android.mk and so doesn't build;
Freescale is actually using a prebuilt binary in a proprietary
repository. With this commit we provide support for building and
installing busybox from sources.

Change-Id: I84adad08ef61a352874ab63154fc67e29cab52e3
Signed-off-by: Diego Rondini <diego.rondini@kynetics.it>