Multiple architecture support in Enterprise BSP

- flash.sh now understands USERSPACE_ARCH
- Move the sdcard mounting/packing into a shell script, capturing
losetup output in Makefile was being flaky for unclear reasons, and
feels really hacky. Also remove the sdcard-allocate target in favor of
allocating in the sdcard.img rule

Change-Id: Ia320c6390e960dd07e377599774afc85ed709143
4 files changed