base-files: flush kernel memory cache during sysupgrade

[Stijn Segers]

Hi,

Have tested this on a few low-RAM devices, among which a TL-WR841N v7 
(32 MiB) and a WNDR3700 v2 (64 MiB).

No idea if this is still necessary/wanted but would be nice to see this 
end up in master (and trickle down to 21.xx :-D ).

Tested-by: Stijn Segers <foss at volatilesystems.org>