sdcard_image-rpi: make rpi-sdimg image dependent on rootfs type

The rootfs may be using a filesystem type other than ext3 so it should
not be hardcoded.

Change-Id: I71c1a96831ce4617b24de7743fe3b70ec5822a33
Signed-off-by: Jonathan Liu <net147@gmail.com>
Merge "sdcard_image-rpi: make rpi-sdimg image dependent on rootfs type"