beaglebone black images too large to fit on eMMC

Created by: chrysn

the images for beaglebone (tested with latest daily freedombox-unstable-free_daily_beaglebone-armhf.img.xz, timestamped 2016-05-27) are unpacked exactly 4gb in size.

in order to flash them directly onto the builtin eMMC (eg. using BBBlfs), they should be smaller -- lsblk -b on the running system reports mmcblk1 to be 386148288 bytes long. (the device is advertised as 4gb, which i'd figure would even mean 4*1024^3 bytes; i have no idea about where the missing bytes are, but at least BBBlfs and the running freedombox seem to agree on the size. there are some /dev/mmcblk1boot[01] devices around which i can't make heads and tails of, but they appear much smaller.)

as a workaround, i flashed the image, edited the partition table to fit, and then resized the btrfs (which to my surprise just worked and did not report errors on the subsequent scrub), but that's something i'd prefer not to rely on.

please consider reducing the image size; even with the decreased size, there's plenty of headroom in the image.

(this was previously reported as https://bugs.debian.org/828839, but that package doesn't exist (yet?)).