flash-kernel is extremely slow on raspi

Currently flash-kernel takes at least 1 and a half minutes (on fast SSD storage) to execute on a Raspberry Pi. This is partly because a considerable number of files are transferred compared to other platforms (all supported dtbs, and their overlays, along with the kernel and initrd which totals several hundred copy operations in all). However, a large part of this is that "maybe_defrag" is called at the end of "backup_and_install".

The "maybe_defrag" function checks the f-k database for the "Bootloader-Has-Broken-Ext4-Extent-Support" flag to determine whether a defragmentation run is required (it's not on Pi models). Unfortunately, it checks this *every time* and the check is fairly expensive (involves bash parsing the f-k database). If this value is cached (as many other values from the database are), this reduces the overall f-k runtime to ~22 seconds (again, on fast SSD storage).