mender-sdimg: introduce MENDER_STORAGE_TOTAL_SIZE_MB, calculate rootfs size
mender-sdimg: introduce MENDER_STORAGE_TOTAL_SIZE_MB, calculate rootfs size

Introduce MENDER_STORAGE_TOTAL_SIZE_MB that specifies the size of the
medium that will hold sdimg image. Rootfs is sized automatically by
distributing the remaining space after subtracting boot & data partition
sizes (incl. partitioning overhader) from the total size.

Fixes: [Mender #596]

Signed-off-by: Maciej Borzecki <f609d9f713abcd5f8f0e51f08dc151638a7681e7@rndity.com>

bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender,bboozzoo/meta-mender