This basic cleanup removes the _ext2/3 overrides from places they
no longer belong since they did not allow further overrides. In doing
this the core-image-minimal* recipes can now set a reasonably small
rootfs so that it's a realistic size for minimal.
The new default for minimal is 8M and will be adujsted upward by the
IMAGE_OVERHEAD_FACTOR (default of 1.3).
This also fixes the ROOTFS_SIZE usage in the IMAGE_CMD_<fstype> code
