sdcard_image-rpi: Use the size of the generated rootfs
sdcard_image-rpi: Use the size of the generated rootfs

When constructing the SD card image, the code was using
the inherited ROOTFS_SIZE, which is the size of the
rootfs contents. When building (for example) a compressed
rootfs, this allocates a partition much larger than necessary.

Instead, take the size of the generated rootfs file that is
about to be written into the generated image, and round it
up to the IMAGE_ROOTFS_ALIGNMENT size.

Fix some comments - the alignment is 4MiB, not 4KiB.

Signed-off-by: Jan Schmidt <def0dbdcd55cbd155ceb01f88cf85ce025345ce8@noraisin.net>
Signed-off-by: Andrei Gherzan <8dc7c09356db0b1762fb140617d7375c0a5a4131@gherzan.ro>

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