mvebu: add sdcard image creation script

mvebu: add sdcard image creation script

Added gen_mvebu_sdcard_img.sh to facilitate creating an fixed-size sdcard image,
adding the bootloader and populating it with actual data.

Added the required rules for creating a 4GB sdcard image according to this layout:
p0: boot (fat32)
p1: rootfs (squashfs)
p2: rootfs_data (ext4)
This should be generic to any mvebu boards that can boot from block storage.

Added the new sdcard image to the Clearfog image profile.

Signed-off-by: Josua Mayer <josua.mayer97@gmail.com>
Signed-off-by: Felix Fietkau <nbd@nbd.name> [cleanup]