Bug 1057970: Don't create SD card partition for Flame devices

The build scripts don't work on Debian Wheezy, because mkfs.vfat is
either hidden from regular users and/or doesn't work correctly.

This patch disables the creation of an image for the SD card partition,
for which mkfs.vfat is required.
Merge pull request #52 from viralwang/bug1057970

Bug 1057970 - Don't create SD card partition for Flame devices. r=tzimmermann